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RADC/NBS International Workshop

Moisture Measurement and Control for

Microelectronics (IV)

Proceedings of the RADC/NBS Workshop

held at the National Bureau of Standards

Gaithersburg, MD

November 12-14, 1986

Didier Kane, Benjamin A. Moore, and E. Jane Walters, Editors

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Electronics and Electrical Engineering

Semiconductor Electronics Division

Gaithersburg, MD 20899

June 1987

This activity was sponsored by:

Rome Air Development Center/RBRE

ss AFB, NY 13441-5700

>nal Bureau of Standards

NBSIR 87-3588

RADC/NBS INTERNATIONAL WORKSHOP

MOISTURE MEASUREMENT AND CONTROL FOR

MICROELECTRONICS (IV)

Proceedings of the RADC/NBS Workshop

held at the National Bureau of Standards

Gaithersburg, MD

November 12-14, 1986

Research Information Center

National bureau of Standards

Gaithersburg, Maryland 20899

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Didier Kane, Benjamin A. Moore, and E. Jane Walters, Editors

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Electronics and Electrical Engineering

Semiconductor Electronics Division

Gaithersburg, MD 20899

June 1987

This activity was sponsored by:

Rome Air Development Center/RBRE

Griffiss AFB, NY 13441-5700

and

National Bureau of Standards

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Director

Table of Contents

Page

Workshop Officers vi

Foreword vii

Introduction .. 1

Opening Talk by S. Hasegawa (NBS) 2

Session I, Chairman, A. DerMarderosian (Raytheon)

1.1: Helium Leah Detector for Small Components

L. E. Bergquist and S. R. Schertz

(Martin Marietta Denver Aerospace) 6

1.2: Permissible Leak Rates and Moisture Ingress

A. DerMarderosian (Raytheon) 15

1.3: The Negative Ion Detection (NID) Vapor Detection Technique: A Breakthrough

in Gross Leak Detection

B. Evan (WEB Technology) 47

1.4: Reusable Moisture Standard Development for Package Gas Analysis

R. F. Haack (California Institute of Technology/Jet Propulsion Laboratory) . 62

Session II, Chairman, R. K. Lowry (Harris Semiconductor)

2.1: Moisture Process Control and Mass Spectrometer Correlations

R.K. Lowry (Harris Semiconductor) 73

2.2: Laboratory Correlation and Survey Experiment: A Status Report

B. A. Moore and D. Kane (RADC) 96

2.3: Accuracy and Reproducibility of the Mass Spectrometer/Carousel Method

for Moisture Analysis

L.J. Rigby (STC Technology Ltd., UK)

Presented by J. Pernicka (Pernicka Corp.) . 104

in
Page

Session III, Chairman, S. D. Senturia (MIT)

3.1: New Failure Mechanisms in Temperature and Humidity Stress

D.Y. Guan, T.F. Gukelberger, E.C. Cahoon,

T.W. Joseph, and J.M. Snowdon (IBM) 124

3.2: Studies of Moisture in Polyimide: A Summary

S.D. Senturia (MIT) 128

3.3: Moisture Control in Hermetic Leadless Chip Carriers with Silver-Epoxy

Die-Attach Adhesive

D.R. Carley and R.W. Nearhoff (RCA) 138

3.4: Polymeric Materials for Moisture Protection of Medical Implants

P. R. Troyk (IIT) 161

Session IV, Chairman, J. Mucha (AT&T)

4.1: Pin Selection Criteria for AC Conduction Technique

N.K. Annamalai and E.C. Blackburn (RADC) and

S.M.R. Islam (Clarkson University) 173

4.2: Evaluation of a Parasitic Charge-Spreading Transistor as a Moisture Sensor

K. Hulten and 6. Hallberg (RIFA AB, Sweden) 189

4.3: Moisture Measurement by AC-Biased Surface Conductivity Sensors

D. Kane and B.A. Moore (RADC) 198

4.4: Tunable Diode Laser Systems for Moisture Measurement in Integrated

Circuit Packages

D. Wall, J. Sproul, and A. Mantz (Spectra Physics) 214

4.5: Using Panametrics Mini-Mod Moisture Sensors for Moisture Measurements

Inside Sealed Microelectronic Packages

D. Pinsky (Raytheon Co.) 254

4.6: Aluminum Oxide Sensor: A Tutorial Update

V. Fong (Panametrics) 276

IV
Page

Session V, Chairman, P. Schuessler (IBM)

5.1: Reliable Plastic ICs for Automotive Electronics

R.J. Straub (Delco Electronics) 277

5.2: A Test Program to Assess Reliability of Plastic Packages and Gel Coatings

for Military Environments

E.C. Blackburn and D. Kane (RADC) 290

5.3: Moisture Monitor for Plastic Packaged Integrated Circuits

H.L. Blaz (Harris Semiconductor)

Presented by R.K. Lowry (Harris Semiconductor) 298

5.4: High Reliability Molded Plastic Packaging Through a Systematic Approach

L. Liang (Fairchild Semiconductors) 307

5.5: High Lead Count/High Reliability Plastic Packaging: Issues and Approaches

G. Pitts (MCC) 319

Workshop Participants

6.1: List of Speakers * 326

6.2: List of Attendees 329

v
WORKSHOP OFFICERS

GENERAL CHAIRMAN: Benjamin A. Moore

Rome Air Development Center

RADC/RBRE

Griffiss AFB NY 13441-5700

(315) 330-4055

VICE GENERAL CHAIRMAN: Stanley Ruthberg

National Bureau of Standards

Semiconductor Electronics Division

Bldg. 225 - Rm A331

Gaithersburg, MD 20899

(301) 975-2082

TECHNICAL PROGRAM CHAIRMAN: Didier Kane(*)

Rome Air Development Center

RADC/RBRE

Griffiss AFB NY 13441-5700

(315) 330-4055

(*) present address: AT&T Bell Laboratories

Rm 3B-126

555 Union Blvd

Allentown, PA 18103

(215) 439-6639

TECHNICAL PROGRAM COMMITTEE

George B. Cvijanovitch

AMP Inc. Winston-Salem, NC

J. Gordon Davy

Westinghouse

Columbia, MD

Aaron DerMarderosian

Raytheon Co.

Sudbury, MA

Didier Kane

RADC

Griffiss AFB, NY

Robert K. Lowry

Harris Corp.

Melbourne, FL

Benjamin A. Moore

RADC

Griffiss AFB, NY

John A. Mucha

AT&T Bell Labs

Murray Hill, NJ

Stanley Ruthberg

NBS

Gaithersburg, MD

Phillipp Schuessler

IBM Corp.

Owego, NY

Steven D. Senturia

MIT

Cambridge, MA

VI
FOREWORD

These proceedings are dedicated to the memory of Stanley Ruthberg, Vice General

Chairman of this 1986 Moisture Workshop, who did most of the groundwork for the

organization of the conference before becoming one of the many victims of cancer.

Stan had co-chaired this RADC/NBS workshop several times in the past and al- ways worked hard to make it the meeting that anybody concerned with moisture

in electronics should attend. Despite numerous tasks at the National Bureau of

Standards, and outstanding contributions to the field of hermeticity, Stan always

managed to devote the time necessary to make this conference a success.

His untimely death, shortly after i;he workshop, is deeply felt by all who knew him,

his colleagues, and friends. We extend our heartfelt sympathy to his family. May

his spirit of intellectual curiosity guided by scientific principles exist in all those

who follow him.

vii
,> v . ,

‘

'

'

*

'
INTRODUCTION

A fourth workshop on Moisture Measurement and Control for Microelectronics was held

at the Gaithersburg, Maryland facility of the National Bureau of Standards on November

12-14, 1986. It was co-sponsored by Rome Air Development Center (RADC) and the

National Bureau of Standards (NBS).

The three previous workshops have been mostly dedicated to the enhancement of measure- ment technologies needed by the microelectronics industry to ensure a higher reliability of

manufactured parts. This year strong emphasis was also put on materials and on packaging. The reason for broadening the workshop in these directions was to help the industry

in identifying moisture-related problems, and in understanding and preventing them at

the design level rather than measuring only their extent.

The 22 selected presentations were centered around 5 sessions (Hermeticity and Standards,

Residual Gas Analysis, Materials Related Issues, Other Moisture Measurement Methods,

and Plastic Packaging), all of which were very well attended. A newly introduced Informal

Workshops Session allowed the participants to discuss topics of their choice with others

sharing the same concerns.

This 1986 workshop has been made possible by:

• Benjamin A. Moore of RADC and Stanley Ruthberg of NBS, who took care of the

general organization,

• Didier Kane of RADC who took care of the Technical Program and the publicity,

• The ten members of the Technical Program committee who selected and reviewed the

papers presented,

• Barbara Lipford and Kathleen Kilmer of NBS who coordinated local arrangements,

• E.. Jane Walters and Josephine Gonzalez of NBS who assisted in the preparation of

these proceedings,

• Sandra B. Kelley of NBS who was responsible for fiscal matters,

• Barbara Lipford of NBS who took care of the registration, and

• Barbara Lipford and Sandra B. Kelley of NBS who assisted in operational details.

DISCLAIMERS

The views and conclusions expressed are those of the authors and do not necessarily represent the official policies of the U.S. Department of Defense, U.S. Department of Commerce,

or the United States Government.

Certain commercial equipment, instruments, or materials are identified in papers published

in this report in order to adequately specify the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of

Standards, nor does it imply that the material or equipment identified is necessarily the

best available for the purpose.

Papers in this volume have not been edited by the National Bureau of Standards. Authors

are solely responsible for the content and quality of their submissions.

1
Definitions of Humidity Terms

Saburo Hasegawa

Chemical Process Metrology Division

National Bureau of Standards

Gaithersburg, MD 20899

(301) 975-2620 or FTS 879-2620

Abstract: The units of humidity, assuming ideal gas behavior, are defined.

The assumption of ideal gas behavior for the mixture of carrier gas and

water vapor should not contribute uncertainties greater than 0.5$ when

making humidity measurements for hermetic semiconductor devices.

Key Words: absolute humidity; dew point; mixing ratio; relative humidity;

specific humidity; volume ratio

1 . INTRODUCTION

The subject of hygrometry is concerned with the determination or measurement

of the water vapor content in a given free volume, occasionally as a separate entity, but more usually as a component of a mixture of gases.

Humidity measurement involves, to a large extent, the basic laws of gaseous

behavior. In many applications, it is sufficient to assume that the mixture

of water vapor with a variety of gases obey the perfect gas laws. In actuality, the mixture deviates from ideal behavior, although at low densities

the departures from ideality tend to a minimum. This assumption of ideal

gas behavior for the mixture of water vapor and carrier gas found in hermetic semiconductor devices should not contribute uncertainties in excess of

approximately 0.5$.

2. DEFINITIONS

As stated above, humidity measurement is the measurement of the water vapor

content in a free volume. When vacuum and/or heat is applied to a material,

free and bound water is released from the material. The total water content is often referred to as the sum of the released water vapor from the

material and the water vapor of the free volume. The amount of water

released from a particular material is dependent on such variables as the

vacuum and temperature applied to the material and the duration of the test.

In general, there is no one-to-one correlation between the measurements of

2
total water and water vapor in the free volume. However, for a specific ar- tifact it may be possible to develop a detailed testing recipe to correlate

the measurement of total water content to humidity.

Occasionally, it may be desired to convert from a reported humidity unit to one which is more descriptive of the environmental condition for a particular application [1], This should pose no problem if the temperature and

pressure of the free volume are reported with the humidity unit. Then the

conversion to other units can be made easily by using the perfect gas laws.

a) Absolute humidity , vapor concentration or vapor density , dv- the ratio

of the mass of water vapory to the total volume of the moist gas V:

v V T R

where e is the partial pressure of the water vapor in the moist gas, T

is the absolute temperature, M is the molecular weight of water, and R

is the universal gas constant.

b) Dew point , t^ - the temperature at which ambient water vapor begins to

condense on a flat surface. The saturation vapor pressure of water at

the dew-point temperature e (t^) is equal to the partial pressure of the

water vapor in the moist gal.

es

( t

d

) = e

The equilibrium humidity in a small volume package can be significantly

altered by an in-situ dew-point measurement. The amount of water vapor

required for the detection of the condensation point can reduce the

amount of water vapor in the small volume and cause an underestimation

of the humidity.

c) Mixing ratio , humidity ratio , mass ratio , r - the ratio of the mass of

water vapor m to the mass of dry gas m present in the moist gas:

v g

P-e M

g

where P is the total pressure and M is the molecular weight of the

carrier gas. ®

d) Relative humidity , U - the ratio of the partial pressure of the water

vapor in the moist gas e to the saturation vapor pressure e at the temperature of the gas:

U = — x 100, % s

3
e) Specific humidity , q

- the ratio of the mass of water vapor m to the

total mass (m + m ) of the moist gas:

v g

r

1 +r

f) Volume ratio expressed as parts per million, ppm-V - the ratio of the

volume of the water vapor to the total volume:

V , , 10° = -p- x 10°, ppm-V

With the use of the above definitions for the units of humidity and by

referencing the measured unit to the temperature and pressure of the

package and reporting the temperature and pressure, the in-si tu humidity can be easily converted to other units with the use of a saturation vapor pressure table for water. Additional information on hygrometry definitions is

given in references [2,3].

3. VAPOR PRESSURE FORMULATIONS

There are a number of formulations for the saturation vapor pressure for

water and ice. The following are a few examples of suitable formulations

for use in most application of hygrometry.

Formulations for saturation vapor pressure over plane surface of water:

Goff-Gratch [4]. The saturation vapor pressure tables for temperatures -50 to 100° C are also published in the Smithsonian

Meteorological Tables , 6th revised edition, Smithsonian

Institution Press, Washington, DC.

Wexler [5]. The saturation vapor pressure tables for temperatures

-50 to 100° C are also published in the ASTM D4230, "Standard

Method for Measuring Humidity with Cooled-Surface Condensation

(Dew-Point) Hygrometer,” American Society for Testing and

Materials, Philadelphia, PA 19103.

Wexler and Hyland [6]. Covers the temperature range 273.15 to

473.15 K.

Haar, Gallapher and Kell [7]. NBS/NRC Steam Tables covering the

temperature range 260 to 2500 K.

Formulations for saturation vapor pressure over plane surface of ice:

Goff-Gratch [4]. The saturation vapor pressure tables for temperatures 0 to -100° C are also published in the Smithsonian

4
Meteorological Tables, 6th revised ed., Smithsonian Institution

Press, Washington, DC.

Wexler [8]. The saturation vapor pressure tables for temperatures

0 to -100° C are also published in the ASTM D4230, "Standard

Method for Measuring Humidity with Cooled-Surface Condensation

(Dew-Point) Hygrometer,” American Society for Testing and

Materials, Philadelphia, PA 19103.

Wexler and Hyland [6]. Covers the temperature range 173*15 to

273.15 K.

4.

DISCUSSION

A set of definitions for humidity units, assuming ideal gas behavior for the

mixture of water vapor and carrier gas, has been presented. If the pressure

and temperature are reported with humidity, the conversions from one humidity unit to another may be made.

REFERENCES

1. Davy, J. G. , Thermodynamic and Kinetic Considerations of Moisture

Sorption Phenomena, Proc. of the NBS/RADC Workshop on Moisture

Measurement Technology for Hermetic Semiconductor Devices, II,

Gaithersburg, MD, Nov. 5-7, 1980, pp. 184-200.

2. Harrison, L. P., Fundamental Concepts and Definitions, Humidity and

Moisture , A. Wexler (ed.), Vol. 3, Reinhold Publishing Corp., NY, 1965,

pp. 3-69.

3. D4023, Standard Definitions of Terms Relating to Humidity Measurement,

American Society of Testing and Materials, Philadelphia, PA 191 03

.

4. Goff, J. A., Saturation Pressure of Water on the New Kelvin Scale,

Humidity and Moisture, A. Wexler (ed.), Vol. 3, Reinhold Publishing

Corp., NY, 1965, pp. 289-292

.

5. Wexler, A., Vapor Pressure Formulation for Water in the Range 0 to 100°

C, A Revision, J. Res. Nat. Bur. Stand. (U.S.), 80A, No. 3, 505 (1976).

6. Wexler, A. and Hyland, R. W. , Formulation for the Thermodynamic

Properties of the Saturated Phases of H

20 From 173.15 to 473-15 K,

ASHRAE Trans., 89, Par.t 2A, 550-519 (1953).

7. Haar, L., Gallagher, J. S., and Kell, G. S., NBS/NRS Steam Tables , Hemisphere Publishing Corp., Washington, DC, 1983.

8. Wexler, A., Vapor Pressure Formulation for Ice, J. Res. Nat. Bur. Stand.

(U.S.), 81 A, 5(1977).

5
1.1 HELIUM LEAK DETECTOR FOR SMALL COMPONENTS

Lyle E. Bergquist and Stephen R. Shertz

Martin Marietta Denver Aerospace

Department 0560

Post Office Box 179

Denver, Colorado 80201

ABSTRACT

A helium leak detector has been developed that detects both gross and fine

leaks in small components. Leaks in a 0.15 cc component are measurable with a hole as large as 0.5 mm in diameter to as small as 10

-12 atm cc / s Qf helium

at 22°C. The unique component in the leak detector system is a modified cryopump that is used both as a roughing pump and entrainment pump for gases other

than helium. Helium, if entrapped, is at a very low level. With the great

range of system sensitivity, bombing of parts can be eliminated in most

cases. Use of 1% helium in the fill gas when the part is sealed will be an adequate fine leak test if tested within a few minutes after being sealed.

Gross leaks can be detected by using the helium in the atmosphere. This system reduces test time and unit costs.

INTRODUCTION

The reliability of small electronic components depends to a great extent upon

how well the component is sealed from reactive gases and water vapor. The

test for determining the hermetic seal is important. The method described in

this paper is a concept where both gross and fine leaks are measured in one

test. This concept uses a modified cryopump where the gases from in the test

manifold are cryopumped and the helium that escapes from the component remains

in a gaseous state. If the leak is a gross leak, the amount of helium is indicated; if the leak is a small leak, the rate of helium coming from the part

is measured. After the measurement is completed, the helium is evacuated from

the system by either a turbomolecular pump or a diffusion pump.

BACKGROUND

Presently, there are several methods for determining the leak rates in small

components. First, the component is placed in a pressurized container and

then backfilled with a gas. The gas is usually helium or krypton. After a specified time and pressure the component is removed and then tested. With

helium, the component may be fine leak tested with a helium-type mass spectrometer or gross-leak tested by submerging the compound in a heated fluid and

6
observing the bubbles that emerge as helium escapes from the component. In

krypton, a radioactive isotope is present and tests are made to determine if

any radioactive emissions can be detected.

There are two other approaches used for gross-leak tests: (1) measure the

weight change; and ( 2 ) measure the hydrocarbon vapor release after pressurizing with a gas.

The bubble test is reported to find only a portion of nonsealed components.

Radioactive krypton is reported to damage memory storage components. Hydrocarbons entering a device can contaminate and reduce the efficiency in certain

hybrid components and this test presently is useful only for gross-leak testing. The major advantages of the method described in this paper are that both

gross and fine leaks can be measured with the same instrument and during the

same test cycle. The unit test costs are reduced as well as the handling

costs. Leaks in a 0.15 cc component can be determined for a hole as large as

0.5, mm in diameter to as small as lO

-

-^ atm cc/s of helium at 22 °C. A rec- ommended operational procedure is to use 1 % helium in the gas when sealing the

component. This will allow leak rate measurements in the 10“® atm cc/s for

many components for a period of a year. Helium pressure bombing of the com- ponents may be necessary to measure leak rates in the 10”^ to 10

- 6 atm

cc/s at 20°C unless the component is tested immediately after being sealed.

Gross leaks can be detected by using the helium that enters the component

along with the atmospheric air.

APPROACH

In this system, a modified cryopump is used as a roughing pump as well as a high vacuum pump. A turbomolecular pump is used to exhaust the helium once it

is measured. A quadrupole gas analyzer is currently being used as a detector,

but a simpler magnetic, sector-type mass spectrometer can also be used.

Figure 1 is a schematic of the leak detector. The operational sequence is as follows

1) The system is evacuated and the cryopump cooled.

2) Vi and V3 are closed and V2 is opened: the purge gas fills the test

volume

.

3) V 3 is opened: the test components are removed and new ones are inserted

in the test volume.

4) Vj_ and V 2 are closed and also V4 : the chamber is sealed and the detector is protected.

5) V 3 is opened: the purge gas is cryopumped. The helium that escapes

from the components under test remains in a gaseous state. All the helium

that escapes from the components under test is contained.

6 ) V 5 is closed.

7) V 4 is opened: the amount of helium in the system is measured or the

rate of rise of helium is determined. From this the size of the leak can

be calculated.

8 ) V5 is opened: the helium is exhausted.

9) V 3 is closed and the cycle is repeated.

7
Figure 1 Schematic of Leak Detector System

The time for a cycle depends upon the size of the manifold in which the component or components are placed for test. If the volume is 5 cc, the time can

be a few seconds. If the volume is 80 cc, a cycle can be completed in about

30 seconds. If the volume is 250 cc, the time is longer, about 2 minutes.

With 250 atm cc of gas the cryopump warms up slightly and must recover its

heat loss. With 5 atm cc of gas the time depends upon the sticking coefficient of the gas on the cryopump and then the actual measurement of the helium. With this system at least 400 atm liters of nitrogen (manifold gas) can

be entrapped in the cryopump before the cryopump needs to be regenerated.

A significant factor in this system is that the sensitivity of the system is

enhanced because the pumping speed is almost zero for helium.

The equation is:

Sensitivity of = Sensitivity of Detector

System Pumping Speed of Trace Gas

If the pumping speed for helium is zero, the theoretical sensitivity for the

system is infinite and the helium will accumulate. This sensitivity may be

limited because a small amount of helium may be pumped by the cryopump. This

amount has not been determined, but is less than an equivalent leak rate of

2x 10“12 a tm cc/s. Also, a small amount of helium may be in the gas that

fills the manifold. In this vacuum system, it is necessary to have low helium

desorption rates. This is accomplished by eliminating all organic seals as well as glass feedthroughs. Metallic gaskets and ceramic feedthroughs are used throughout the vacuum system.

This system measures either the amount of helium or the rate in which the

helium is released from the component. If the leak is gross, all the helium

will have escaped from the package during the evacuation and the amount will

be detected. This will be measured as a flat line. If the leak is small, the

rate in which it escapes will be measured.
Figure 2 is a graph of some leak rate possibilities. Components having large

leaks in small packages will be exhausted in one second or less since no heli- um pumping is occurring during the time. The turbopump is valved out of the

system. The output from the detector in amperes will indicate a constant

level. With smaller leaks the output will indicate a rate of rise versus time

by using a calibrated leak and recording this rate of rise.

Figure 2 Helium Leakage Measurement

The measurement of low-level helium levels requires a mass analyzer that can resolve HD with a mass-to-charge rate of 3 to helium with a mass-to-charger

rate of 4. Figure 3 is a graph of these two masses. The resolution is adequate so no interference comes from mass 3 while monitoring the helium peak.

Figure 3 Resolution between M/e 3 and M/e 4

Figure 4 is a typical rate of rise from a calibrated helium leak with a leak

rate of 7.6x10“-^ atm cc/s at 20°C. Using this measurement, it can be com- pared with other rates of rise to determine the leak rate of other components

under test.

9
100.0

,-11

I I I I 0 15 30 45 60

Time, s Figure 4 Calibration of System Valve to TurboMolecular Pump Closed

Figure 4 shows the result when a 0.10 cc volume with a conductance of 1.5 cc/s

is measured. Only helium in atmospheric air was used for this measurement.

The line is straight, which indicates that all the helium escaped the 0.10 cc volume when the backfill gas in the manifold was cryopumped.

Range: 10 ^ A

2.0 r1.0 -

30 60

Time, s Figure 5 Calibration of System Using 0.1-cc Volume

with Conductance 1.5 cc/s Helium in Air

Figure 6 shows the helium background caused by having a glass-enclosed ion

gauge in the system. This particular gauge had a helium leak equivalent to

1.6x 10~12 atm cc/s of helium at 22°C. Ion gauges of this type have helium

leakages from 2x10“^--*- to about lxlO

- -^ atm cc/s of helium. This device is

used to see if helium is being cryopumped in the system.

10
1.0

Range: 10 ^ A

Slope

Equivalent Leak Rate

1.6x1 O' 1

2

atm cc/s of He at 22°C

I I

15 30 45 50

Time, s Figure 6 Helium Entering System from Glass Ion

Gauge

Figure 7 shows a graph of the helium in the system when the ion gauge is

valved out of the system. The rate of rise is 6xl0~-*-^ atm cc/s of helium at

22°C.

Helium Background

Ion Gauge Out of System

Valve to Turbo Closed

Figure 7 Helium Background in System

Figure 7 is an example of testing T05 transistor packages with a different

size hole in each package. These holes were 0.2, 0.3, and 0.4 mm in diameter. In a package with 0.8 mm, the air escaped while inserting it into the

manifold. In these tests only atmospheric air and the helium in the air was used

.

11
Gross Leak

Figure 8 Gross Leaks in T05 Transistor Package

Using Helium in Air

Figure 9 shows the findings when a hybrid package was tested. The dimensions

of this package were 2x2xl/4 in. and were backfilled with 10% helium, with the

remainder being nitrogen. The free space in the hybrid was about 3 cc. The

volume of the manifold was nearly 300 cc and a little helium from the air entered. If the leak was gross, the measured output would have been several

decades higher. A stabilizer plate was placed over the sealed plate with

epoxy and some air bubbles could have come from the epoxy. Air bubbles were

detected in a previous test using the bubble test method.

Time, s Figure 9 Test for Gross Leaks in Package 2x2xl/4 in.

12
Figure 10 shows fine leak testing the same hybrid by evacuating the helium

from the previous measurement and detecting the rate of rise of helium coming

from the hybrid package. This rate was less than 10”-^ atm cc/s of helium

at 22°C. The requirement for this component was less than 10“^ atm cc/s

for helium.

FW-79

Fine

-1

1

Range: 10 A

Figure 10 Fine Leak Test in Package Sealed with

10% Helium Package 2x2x1/4 in.

ANALYSIS OF FINDINGS

The system has a sensitivity range to easily measure leak rates filled with 1%

helium that have actual leak rates of 1x10“° atm cc/s at 20 °C. Gross leaks

with holes as large as 0.4 mm in diameter can be detected in 0.15 cc components. This can be determined by detecting the helium that entered the component from the atmospheric air. If 0.1 cc component remains in atmospheric

air for more than 2 hours, bombing with helium will be necessary to measure

leak rates in the 10“^ - 10“ ^ atm cc/s region.

Bubbles were observed in the hybrid package during the gross leak checking

process using the bubble test method. However, it was proven that these bubbles came from the epoxy and not from the sealed hybrid. The hybrid was later

tested by opening it and had 10% helium with the nitrogen. This was' a costly

hybrid design and saved the manufacturer from scraping the unit.

13
CONCLUSIONS

A helium leak detector has been designed that can test for both gross and fine

leaks. Either the helium that has escaped from the component is measured or the rate in which it escapes is measured. With this detector system most com- ponents can be tested if the component is filled with 1% helium during the

sealing process and immediately tested. Otherwise, bombing with helium is

necessary. This saves several operations during the leak checking process.

REFERENCE

Bergquist, L. E.: "A Helium Leak Detector for Small Components," in 23rd

Annual Proceedings Reliability Physics, 1985, pp 65-67.

14
1.2 PERMISSIBLE LEAK RATES AND MOISTURE INGRESS

Aaron DerMarderosian

Raytheon Company

Equipment Division

Sudbury, MA 01776

ABSTRACT

A guideline is presented which addresses the maximum leak rate

acceptable for hermetically sealed devices as a function of internal

package volume and moisture content.

The guideline is based on work detailing the rate at which water

vapor penetrates packages with known leak rates and calculations on

the quantity of water needed to form an ionically conductive film on

the internal surfaces of the package. The results indicate in general

that present standards are far too lenient.

j i

15
Introduction

A review of the literature dealing with packaged semiconductors reveals that a large percentage of the failures are

1-8 associated with one type of corrosion mechanism or another.

Moisture has, and continues to play a large part in this mode

of failure. The sources of moisture in these packages have been

9-11 delineated in various papers and may be categorized as follows:

1) entrapped water due to poor bake out procedures 2) generated

internally by decomposition, material outgassing or devitrification

of sealing glasses 3) permeation thru polymer seals and 4) penetration

of cracks and/or faulty seals. It is this latter area which is

probably the least well understood and has, in part, been due to the

lack of published information on this subject matter. Some work in

2 2 TO

this area has been published by Davy ' and Stroehle highlighting the inadequacies of present leak testing practices and

specifications. The work by Davy was essentially limited to a theoretical treatment while the work of Stroehle combined a theoretical discussion with the experimental failure rate of a specific part

type r, igh humidity to its leak rate as measured on a helium leak

detector. The purpose of this work is to add to this data base and

provide for some rationale in the specification of maximum allowable

leak rates for hermetically sealed devices. The study focuses, experimentally, on the rate of moisture penetration into cavities (at zero

total pressure difference as one might expect at normal ambient conditions) as a function of leak rate measured during standard test conditions of 15 psi pressure differential. The results of these experiment

are then compared to a theoretical analysis using established gas flow

equations typically used in vacuum technology.

16
Experimental Studies

Gross Leak Range (1Q“^ to 10~5 atm cc/sec)

The test samples used in this portion of the study were

fabricated using a standard TO-5 transistor can fitted with a 1 mm

glass capillary tube and a miniature surface conductivity type

moisture sensor.

The capillary tubes were intially exposed to a flame at one

end until melting occurred. This heating subsequently reduced

the I.D. of the tip of the tubing, thereby providing for a method of producing leakers over the range of interest (see Figure

1) . Tubes fashioned in this manner were then calibrated by using

the rate of pressure rise method, a technique which determines the

air leak rate at standard conditions of 15 psi differential.

Briefly, the procedure is as follows; tubes are connected to a container of a known volume at a known pressure (usually 1 torr)

.

Air is then allowed to leak through the capillary and into the

container. The time required to raise the pressure to 2 torr (or

10 torr depending on the leak range) is measured and used to

calculate the air leak rate for the capillary using the gas law

equation (PV = MRT) . This equation is used to determine the

quantity of gas which leaks into the container, and when divided

by the time in seconds required to raise this pressure to the set

amount (2 torr or 10 torr), yields the air leak rate in atm cc/sec

(see Figure 2)

.

This method is cumbersome and time consuming, however, and

an alternate quicker method was employed to calibrate the remainder

17
of the capillaries. The technique is as follows; the calibrated

end of the tube was submerged in FC-77 liquid while the other

end was connected to a source of nitrogen gas. The gas pressure

was slowly raised until bubbling was initiated at the leaky end.

A graph was then constructed which depicts the differential

pressure required to initiate bubbling as a function of air leak

rate (see Figure 3).

Each of the capillary tubes which were calibrated with this

faster method were then sealed to the cover of a TO-5 can containing

a calibrated moisture sensor (see Figures 4 & 5)

.

The moisture sensors were calibrated prior to sealing by

exposing them to various saturated salt solutions of known relative

humidities at room temperature and monitoring the change in surface

leakage at each of these conditions (see Figure 6).

Test Procedure and Results . The completed test samples were placed,

leaker side down, thru a test plate pre-drilled to accept the TO-5

package cover. The parts were then wax sealed at the edges and

then exposed to a relative humidity (saturated salt solution) of

80% at room temperature (^-22°C average). The rate of moisture

penetration was monitored as a function of time with each of the

surface conductivity sensors. Prior to the test the parts were

stored for 1 month in an ambient environment of 15% RH @ 22°C,

thereby establishing an initial water vapor pressure of ~3.4

torr within the package.

The parts remained in the 80% R.H. ambient until equilibrium

was reached for the smallest leakers. The time constant for each

of the devices was then obtained from this data and ranged from

18
zv 0.6 hours (10 ^ atm cc/sec, air) to *j 700 hours (10“^ atm

cc/sec, air) . From these time constants the "water vapor" leak

rate in cc/sec* was calculated using the following:

JlJ Q * V Where: Q « water vapor leak rate in cc/sec.

V - volume of package in cubic centimeters

(TO -5 = .17 cc)

T = time constant (time required to reach

63% of equilibrium pressure) in seconds.

The values of these time constants and the water vapor

leak rate data are plotted in Figures 7 and 8.

Fine Leak Range (10~^ to 10 atm cc/sec)

The test samples used for this portion of the study were

fabricated from 14 lead, 1/4 X 1/4 inch ceramic flat packages. The

samples contained surface conductivity moisture sensors of lower

sensitivity than those used in the gross leak study (only 2

parallel strips) and were calibrated in a manner similar to the

method used in the TO-5 study. The leakers were generated by

thermally shocking the devices from +200°C air to -196°C (liquid

nitrogen) . The shapes of the leaks created by this method were

found to be cracks in the seal area in and around the leads. The

leak testing of the devices was performed by using method 1014

of MIL-STD-883 B (flexible method A - Helium). The leak rates

a

were established through the use of computer generated curves which

* A cc of water vapor is defined here as that quantity of moisture

contained in one cubic centimeter of air at atmospheric pressure

and 22.2°C when the partial pressure of water vapor is 16.1 torr

O 80% R.H. @ 22 . 2°C)

.

19
plot the indicated leak rate (R) as a function of air leak rate

(L) and take into account the volume of the part and the various

bombing parameters'^ (e.g. pressure, pressurizing time, etc.).

All parts were tested on the leak detector at least twice at known

intervals to help establish the leak rate more accurately e.g.

typically an indicated or measured leak rate has two points on the

curve, one of which corresponds to the lower end of the spectrum

while the other point corresponds to the higher end (see Figure 9).

To determine which of the two possible leak rates it is, another . measurement is necessary at some period of time later (typically

1-2 hours). If the second reading remains the same as the first,

then the leak rate is determined from the fine leak end, conversely

if it is significantly less, then the leak rate is determined from

the higher leak end.

Approximately 25 pieces were measured to have an air leak

rate in the 10 ^ atm cc/sec range; another 16 in the 10 ^ atm cc/sec

- 8 - 9 range; 10 in the 10 atm cc/sec range and 1 in the 10 atm cc/sec

range

.

Test Procedure and Results . The completed samples were placed into

a dessicator jar which contained a saturated salt solution identical

to that used in the gross leak study (80% RH @ 22°C). The devices

were periodically removed from the chamber and monitored for

moisture content by measuring the change in surface leakage on the

moisture sensors. Prior to this test, the devices were subjected

to an ambient of ?2 O

5

for a period of three weeks. It is conservatively estimated that the parts reached an internal dew point

20
of -15°C (water partial pressure of a# 1.4 torr) under these

conditions

.

The parts remained in the 80% RH ambient until equilibrium

was reached for the smallest leakers (as noted by the sensors) the

time constant for each of these devices was then obtained from

this data and ranged from a low of a* 10 hours (10 range) to a high of a* 280 hours (10“® range). The water vapor leak rates

were calculated as before (equation 1) using an internal volume

of 0.008 cc for these devices. The values of these time constants

and leak rate data are plotted in Figures 10 and 11.

The performance of these devices were markedly different from

the performance of the capillary leakers used in the gross leak

tests. The main difference was the wide variation in equilibration

times noted between parts with the same air leak rate. In some

cases, varying by nearly an order of magnitude I This type of

performance possibly suggests that the leak rate may be affected

by pressure differentials either during test and/or during the

helium "bombing" environment of 75 psig.

Another likely possibility may have been capillary condensation

within the crack which may have altered the penetration rate significantly. This effect was substantiated somewhat by post humidity

leak test measurements. Approximately 35% of the devices had

indicated that they had been plugged sometime during the test

while another 15% indicated a change in leak rate. A subsequent

bake @ 150°C for 24 hours returned many of the parts to their

original values yet nearly 4% remained plugged while another 25%

never returned to their former leak rates.

21
A greater understanding of the behavior of these irregular

leak paths is necessary before much more can be concluded from

this data. It is clear, from the short time constants measured,

that water vapor can penetrate so called fine leakers and, if these

parts are typical, it becomes obvious that present specifications

(hIL-STjJ-883B) for maximum allowable leak rates are too lenient

for high reliability devices.

Theoretical Studies

Gross Leak Range (10 ^ to 10 ^ atm cc/sec)

A review of the fundamental gas flow equations'^ delineates

three definable areas of flow regime; namely viscous, transitional

and molecular each of which is described below.

Viscous Flow

Viscous flow occurs when the mean free path of the gas is

smaller than the cross-section dimension of the physical leak.

Poiseuille r s Law for viscous flow through a cylindrical tube is

shown

.

Q = IT r

A

8 nl

Q

= X r

4

16 nl

where

Q

= flow rate in micron liters/sec

X 10

-3

P

a

(

.P

1

- P 2> [2]

or

- 3 2 2

X 10

J

p

1 - p2 [ 3 ]

22
r = radius of tube in centimeters

n = viscosity of gas in poise

1 = length of tube in centimeters

P = average pressure P, + P in microns

a jl &

2

p = upstream pressure in microns

P = downstream pressure in microns

Transition Flow

Transition flow occurs when the mean free oath of the gas is

approximately equal to the cross-section dimension of the leak and

it occurs under conditions intermediate between viscous flow and

molecular flow. Again for a long tube, the flow may be expressed

as shown below. As can be seen, the expression is more complex

than that for the other types of flow. Note that the portion

outside the brackets is identical to the expression for molecular

flow

.

where

Q = flow rate in micron liters/sec

r = radius of the tube in cm

1 « length of the tube in cm

M = molecular weight of gas in amu

P^ = upstream pressure in microns

- downstream pressure in im microns

T » temperature in °K

Q _ 30.48 r

3

1 +

1 + 2.507

1 + 3.095 H

23
L = mean free path in cm at the average pressure

a P

1

+ P

2

2

Molecular Flow

Molecular flow occurs when the mean free oath of the gas is

greater than the longest cross-Section dimension of the physical

leak. Knudsen’s Law for molecular flow neglecting the end effect

is shown,

where

Q -

(

p

> - p

2 ) H

Q = flow rate in micron liters/sec

r = radius of tube in centimeters

1 = length of tube in centimeters

M =» Molecular weight of gas in amu

P^ = upstream pressure in microns

= downstream pressure in microns

T = absolute temperature in °K

Each regime has its own equation describing the effects of the

geometric considerations of the leak path along with various gas

parameters. An examination of these equations and methods used

to determine flow characteristics (Figure 12), suggests that

leakers in the gross leak range (10“^ to 10”^ atm cc/sec) fall

into the viscous and transitional flow regime under standard

test conditions (15 PSIA on one side of the leak and 0 PSIA on

the other side of the leak) . Using these assumptions and a leak

length path of 0.6 mm, calculations of the air leak rate as a function of hole diameter at standard conditions were performed

and are shown graphically in Figure 13. Knowing this "effective”
diameter of the capillary leaker enables one to then calculate

the water vapor leak rate as a function of the hole diameter

under any of the gaseous flow regimes using the equations given.

Since', in the case of our experiment with the capillary leakers,

there was no difference in total pressure between the inside and

outside of the device during humidity exposure, we chose the molecular flow equation to calculate the rate of moisture penetration and

plotted the results in Figure 14. In addition to this, the results

of calculations for water vapor penetration using the Combined Flow

Equation of Davy^ were plotted with it. The two equations differ

considerably at the larger diameters (^2 orders of magnitude) yet

join together and agree closely at the smaller diameters and are

essentially the same thereafter. There is no intent here to suggest

which equation is more correct, rather they are offered as a point

of discussion to help place the problem within certain boundaries.

Calculations of water vapor leak rates at a partial pressure

difference of 12.7 torr , as a function of air leak rates (15 psi

pressure differential) were then performed using the molecular

flow equation. These results along with the results of experimental data are shown graphically in Figure 15. Again, the results

of calculations taken from the Combined Flow Equation are presented

with it for the sake of comparison. It can be seen from this

graph that the experimental data tends to agree more closely with

the molecular flow equation at the higher leak rates while either

one of the equations adequately describes the moisture penetration

rate at the lower values of air leak rates.

25
Finally, calculations were performed @ 80% R.H. (molecular

flow eq.) which indicate the rise in dew point of the TO-5 package

vs time as a function of air leak rate. The results are shown in

Figure 16.

- 6 _ s Fine Leak Range (10 to 10 atm cc/sec

The theoretical considerations in this portion of the study

were simplified somewhat because of the general agreement among

12 18 various investigators ’ in that the predominant flow mechanism

in the fine leak end of the spectrum is molecular. This agrument

is further strengthened by analysis of the geometry of leaks

(essentially thin cracks) and their approximate dimensions (on the

order of 10 ^ cm or less in thickness and up to ~ 1/2 a cm in

13 length). A detailed analysis of this subject is given by Davy

and is recommended for those wishing to pursue this matter further.

In performing the calculations for the fine leak spectrum the

following assumptions were made: the flow was molecular and the

gas passed through a single capillary hole of "equivalent effective"

diameter (0.9mm long ). The results of calculations of hole

diameter as a function of std. air leak rate are shown in Figure 17.

Knowing this "effective" diameter we then calculated the water vapor

leak rate as a function of ho^e diameter. The results of these

calculations are shown in Figure 18 and are combined with the

experimental data, generated earlier, in Figure 19. Examination of

the latter graph indicates a significant difference in the 10 ^ to

- 8 10 atm cc/sec range between the experimental and theoretical

26
data while reasonable agreement exists in the 10 ^ range. It is

not known at this time why such a variance exists between the

data. Several areas are suspected and will be addressed in

follow-on experiments.

Finally, a series of calculations were performed which

determined the water vapor penetration time constant for several

typical package types as a function of standard air leak rate.

The results are shown in Figure 20 and highlight again the leniency

of leak rate specifications and the need for more definative limits

for high reliability applications.

Discussion

Many fundamental questions relating to the rate of moisture

penetration into packages with known air leak rates have been

answered during this study. Although some questions remain,

the author feels that sufficient data was obtained to establish

analytical methods that address this issue. It is interesting

to note that the test results of this program support the basic

13 14 premise of Davy and Stroehle , that moisture rapidly penetrates

- 8 devices with leak rates as low as 1 X 10 atm cc/sec.

Gross Leak Range (10 ^ to 10 ^ cc/sec)

The results of this portion of the study were encouranging to

say the least, considering the close correlation obtained between

the experimental and theoretical data. Regardless of this agreement,

however, it was perfectly clear that the water vapor penetrated

these small diameter holes very rapidly, thereby resolving long

standing questions relating to this issue. Another frequently

raised question is that of capillary plugging due to condensation

27
in small diameter holes. The glass tubes used in this study were

very informative with regard to this matter. Approximately 20%

of the devices were plugged with water almost immediately after

exposure to the high humidity atmosphere. Optical inspection of the

_2 larger leakers ( 10 cc/sec range) revealed that liquid water

was trapped in the narrow passageway. Subsequent heating in a flame, however, quickly removed the moisture and exposed small

quantities of contaminants believed to be responsible for the

initial absorption of moisture. When these tubes were returned

to the high humidity conditions they allowed water vapor to pass

through at a normal rate. It is felt that the rapid heating and

high temperature obtained with the flame altered the behavior of

the contaminants thereby allowing the moisture to pass through.

Since all of the plugged devices could be "cleaned" by flame

heating, it was concluded that the condensation was not related to

true adsorption on the walls of the capillary.

Fine Leak Range (10 to 10~ atm cc/sec ) The deviation noted between the experimental and theoretical

data obtained in this part of the study was somewhat discouraging

at the outset. Considering the complexity of the leak path geometry,

however, and the method of determining their leak rates this could

be expected. The author feels that a follow on study addressing

these issues in greater detail, will answer most of the questions

raised during this experiment. It is important to note and stress,

however, that water vapor did penetrate those parts with leak rates

as low as 1 X 10 cc/sec in a relatively short period of time.

28
As mentioned earlier in the test results , many of the fine

leakers indicated that they had plugged during exposure to the high

humidity atmosphere, and that most of them returned to their original

leak rates after heating. Since visual observation of the leak paths

was not possible (cracks widths in the order of 10 ^ cm) little

could be learned about the details of the plugging and if contamination

was a factor. The fact that some parts ('^4%) were plugged "permanently" and that others ( 25% did not return to their original

leak rate suggests that true capillary condensation may have occurred

and/or the leak pathway had been altered. The unrepeatable behavior

of these parts suggests that the cause of problems encountered with

leak test correlation studies may be related to ambient humidity levels

or handling seen at the testing facilities or during transport.

Leak Rate Specification*

In view of the results of the moisture ingress study, a set

of guidelines were developed which can be useful for those involved

in component specification and procurement. The guidelines were

developed by first examining the present military standard (MIL-STD883B) which specifies maximum allowable leak rate and maximum

allowable package ambient moisture content. Assuming that a package is dry when sealed, a series of calculations were made

utilizing the moisture ingress equations developed in our earlier

work. The following equation defines the moisture ingress time

constant of any size package as a function of its standard air leak

*The reader is encouraged to examine the work by others who have

addressed this same issue . 1 2 , 1 3 , 1 9 , 20

29-
rate and is given as:

T = -011V

Qa

where T = Time constant in hours for ^0 vapor ingress (63%)

V = Internal volume of device in cubic cent.

Qa

= Air leak rate in std. cc/sec

This equation was used to calculate the time required to exceed

a moisture level of both 5000 ppmv (present limit for Method 1013,

MIL- STD- 883B) and 500 ppmv (proposed future standard 1988-1990) for

packages with internal volumes ranging from .009 to 5cc. The information was then compared to the maximum allowable leak rates as

specified in method 1014 of MIL-STD-883B fine leak test helium and

krypton 85.. Figures 21 through 23 show the results of these calculations and dramatically highlight the inadequacy of the fine leak

test specifications for the greatest majority of today's packages,

rhese inadequacies are shown in three major areas: 1) The helium

"fixed" method (Test Condition A-j) is inconsistant with the "flexible"

method (Test Condition A£) i.e., reject limits vary by a factor of

2 to 1 . 2) The helium method i§ far more lenient than the krypton

method (Test Condition B) i.e., reject limits are as much as 40 times

nore stringent, especially for larger volume packages and 3) None of

these test conditions are adequate with respect to moisture ingress

rates; especially for the lower volume packages ('v 1 cc)

.

Although

30
the specification leaves much to be desired, it at least attempted

to address the issue at that time ( ~1968 - 1977). Prior to

the issuance of this specification, leak testing procedures were

much more arbitrary and even more poorly executed.

In an attempt to update this specification and to provide

the reader with a rationale for specifying a leak rate for high

reliability semiconductor devices , a model was developed using the

following assumptions:

1. The critical minimum water layer thickness which can support

ionic transport is three (3) monolayers or 1.2 X 10"^ cm

(1.2 X 10 ^ gms/sq. cm).

2. The leak rate of the device does not change with time or

temperature

.

3. The package internal surfaces have a roughness factor of unity

4. There is no absorption into the package materials (i.e., such

as plastics or dessicants) nor any chemical reactions which

would reduce the moisture level.

5. All of the moisture which leaks into the package is available

for condensation and that it does not freeze.

6. The water condenses uniformly on all internal surfaces.

7. The package is equal in length and width and the thickness is

one fifth of the length.

8. No serious consideration was given to non-uniform condensation

This is a rather lenient position taken on the part of the

author and will be addressed at a future date.

31
Using the assumptions of item 7, a table was generated

detailing the package dimensions, internal surface area, internal

volume, and the ratio of the area to volume (see Figure 24). From

this data and the three (3) monolayer threshold (Item 1) another

table was generated which establishes the maximum allowable moisture

-3 2 content for packages with internal volumes ranging from 10 to 10

cubic centimeters. These values of maximum moisture level range from

13,000 ppmv for the smallest package to 280 ppmv for the largest

package! This might be surprising since some may intuitively feel

that the numbers should be reversed. Examination of the role of

the ratio of internal surface area to internal volume and the

absolute moisture content, however, will quickly subdue these

"feelings" (see Figures 25 and 26). Briefly, the high ratio of the

surface area to volume in the smaller packages allows for more

surface proportionately for moisture to condense on, hence, allowing

for a more tolerant moisture content and vice versa for the large

volume package with its small ratio of surface area to volume.

Examination of Figure 26 reveals the rather curious or possibly

ironic piece of information concerning the present moisture limit of

method 1018 in MIL-STD-883B. At the intersection of the dotted

horizontal line at 5000 ppm^ and the three (3) monolayer sloped

line, it can be seen that most D.I.P. and flat packages with volumes

that range from 0.01 to 0.05cc have a limit of moisture content that

is within ^-20% of the current limit!

Finally, a series of calculations were made which involved the

determination of the moisture ingress time (at a 13,000 ppmv external

32
ambient) as a function of air leak rate using the three (3)

monolayer limit discussed earlier. The information was obtained

graphically by using Figure 27, the moisture ingress time

displayed in an exponential curve as a function of time constant

and the external ambient moisture level in ppmv . These calculations appear in Figure 28. It should be noted that the .OOlcc

volume has three (3) monolayer package moisture value of 13,000 ppmv

(Figure 25) and that five (5) time constants (T) were used to obtain

the values for this package to reach this level. This departure from

the straight line relationship is graphically illustrated in Figure

29. Further examination of Figure 29 again underscores the

inadequacy of present leak rate specifications for "hermetic" devices

with an expected life of £ 10 years. In addition, it is apparent

that for packages with an internal volume of less than /v one (1) cm3,

- 8 the air leak rate should not exceed 1 x 10 atm cc/sec near the

3 - 9 3

one (1) cm range and 1 x 10 atm cc/sec near the 0.03 cm range!

Considering what the industry in general has been testing to, this

is not welcome news. There is, however, some encouraging data from

those who have examined overall leak rate distributions in devices.

It has been noted that most devices which do not exhibit leakage

- 8 greater than 1 x 10 atm cc of air/sec, tend not to leak at all.

This is primarily due to the nature of defects in this size range,

i.e., they are simply extremely difficult to produce intentionally

or otherwise. This is no cause for complacency, however, since there

33
are other issues, such as crack propragation , package strength, etc.,

which could override these factors. This set of rather complicated

factors suggests that packages and their resultant state of hermeticity

should be carefully designed to assure initial and long term integrity.

In the final analysis, the burden for this task lies with the packaging

industry and those of us involved in reliability issues should exert

the pressure necessary to accomplish it.

One important issue yet to be resolved is the correlation

between moisture levels in the package and time to device failure.

Although beyond the scope of this paper, the author feels that this

question may be answered best by performing operational tests on

open devices during exposure to humidity conditions. Various

acceleration factors may be obtained from this data and be used to

predict life times of devices with known leak rates at any given

ambient humidity. Once established, this data could then be used

along with other factors to help guide those involved in procurement

specifications which address these types of reliability issues.

Conclusions

Based on the results of this study, several conslusions may be

drawn. Some of the more important ones are as listed below:

1. Water vapor rapidly penetrates typical integrated circuit packages

- 8 with leak rates as low as 1 X 10 atm cc/sec.

2. The rate of penetration of water vapor into packages which

leak may be approximated by assuming molecular flow.

34
3. The maximum allowable leak rates specified in MIL-STD-883B

(Method 1014) are too lenient in general and are especially

so when considering the maximum proposed moisture level

specification of Method 1018 (500 ppm).

4. The "fixed" methods of fine leak testing in MIL-STD-883B

should be eliminated, allowing only for the flexible method.

5. The maximum allowable leak rate for all packages should be less

- 8 than 1 X 10 STD cc of air/sec.

35
References

1. "Principles of Corrosion" S. Kolesar; 12th Annual Proceedings,

Reliability Physics 1974 pages 155-167.

2. "Metallization Corrosion in Silicon Devices by Moisture Induced

Electrolysis" H. Koelmans; 12th Annual Proceedings, Reliability

Physics 1974 pages 168-171.

3. "The Effects of Phosphorous - Doped Passivation Glass on the

Corrosion of Aluminum", W. Paulson and L.W. Kirk; 12th Annual

Proceedings, Reliability Physics 1974 pages 172-179.

4. "Temperature Humidity Acceleration of Metal - Electrolysis

Failures in Semiconductor Devices", D.S. Peck and C.H. Zierdt

,

Jr. Bell Telephone Laboratories Report.

5. "Failure Analysis Techniques Applied in Resolving Hybrid

Microcircuit Reliability Problems", G.H. Ebel 15th Annual

Proceedings, Reliability Physics 1977.

6. "Migrated-Gold Resistive Shorts in Microcircuits", A. Shumka

and R. Piety, 13th Annual Proceedings, Reliability Physics 1975.

7. "Further Studies on the Reliability of Thin-Film Nickel-Chromium

Resistors", W. Paulson 11th Annual Proceedings, Reliability

Physics 1973.

8. "Reliability Technology for Cardiac Pacemakers" NBS Special

Publication 400-28, Editor H.A. Schafft, 1976.

36
9.

"Moisture, Myths and Microcircuits", R. Thomas, Proceedings

26th Electronic Component Conference, 1976 page 272.

10. "Nichrome Resistor Investigation", A. DerMarderosian , Raytheon

Internal Report, IS: 72: 6 dated 26 January 1972.

11. "Non-Hermeticity of Polymeric Lid Sealants", R. K. Traeger

,

IIEE Transactions on Parts, Hybrids and Packaging VOL PHP-13

No. 2 June 1977.

12. "Model Calculations for Maximum Allowable Leak Rates of

Hermetic Devices", J.G. Davy, J. Vac, Sci. Tech. Vol 12 No. 1

Jan/Feb 1975.

13. "Calculations for Leak Rates of Hermetic Packages", J.G. Davy,

IIEE Transactions of Parts, Hybrids and Packaging, Vol PHP-11

No. 3 September 1975.

14. "On the Penetration of Gases and Water Vapor Into Packages

With Cavities and a Maximum Allowable Leak Rates" D. Stroehle,

Reliability Physics Symposium Proceedings 1977.

15. MIL-STD-883B , Helium Test Leak Rate Equation Method 1014.

16. C.E.C. Leak Detection Manual (//992249-0011 ) Pages 2-2, 2-3.

17. MIL-STD-883B , Method 1014.

18. "Back Pressurizing Technique of Leak Testing", D.A. Howl and

C . A. Mann, Vacuum, 15, 347 (1965).

19. "Guidelines for Sealing and Leak Testing of Hermetic Packages",

J. Gordon Davy, Editor Westinghouse Defense and Electronics Center

Baltimore, MD. Internal Report.

37
20. "The Objectives and Limitations of the Hermeticity Tests

Performed on Semiconductor Encapsulations". R.P. Merritt,

British Telecom, Internal Memorandum No. R20/014/83.

38
PRESSURE

IN

PSI

CALIBRATED GROSS LEAKER

CONSTRUCTED FROM 1 MM CAPILLARY

Figure 1

CALIBRATION OF CAPILLARY LEAKERS

(RATE OF PRESSURE RISE METHOD)

COMPLETED ASSEMBLY OF CALIBRATED

fiPflSS IFAKFR ANfl Tfl-R HAM

Figure 4

SURFACE CONDUCTIVITY

MOISTURE SENSOR

DIFFERENTIAL PRESSURE REQUIRED TO INITIATE

BUBBLING VS AIR LEAK RATE OF CAPILLARY

l00 l (FC-77 LIQUID)

IO-=

Figure 5

Figure 3

39
HYGROMITE CALIBRATION RELATIVE HUMIDITY VS

SURFACE LEAKAGE CURRENT

AIR LEAK RATE VS WATER VAPOR LEAK

Figure 6 Figure 8

TIME CONSTANT FOR WATER VAPOR

PENETRATION VS AIR LEAK RATE FOR TO-5 CAN

Figure 7

MEASURED LEAK RATE VS AIR LEAK RATE

Figure 9

40
WATER

VAPOR

LEAK

RATE

IN

CC/SEC

TIME

IN

SECONDS

(T)

TIME CONSTANT FOR WATER VAPOR

PENETRATION VS AIR LEAK RATE FOR

1/4 x 1/4 FLATPAK (V0L.-0.008CC)

FLOW CHARACTERISTICS

\

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Ui

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1 iiiit) 5 ,„-2 .«-• ,~0 ,„i .„4 ,J ,„6 PRESSURE IN MICRONS AIR AT C Figure 10 Figure 12

AIR LEAK RATE VS WATER VAPOR LEAK RATE’

Figure 11

AIR LEAK RATE VS HOLE DIAMETER

FOR CAPILLARY LEAKERS

Figure 13

41
HOLE

DIAMETER

in

microns

WATER VAPOR LEAK RATE VS

HOLE DIAMETER FOR CAPILLARY LEAKERS

Figure 14

AIR LEAK RATE VS WATER VAPOR LEAK RATE

Figure 15

CHANGE IN DEWPOINT VS TIME AS A FUNCTION OF STO.

AIR LEAK RATE IN AMBIENT ENVIRONMENT OF 80% R H. AT 22 2°C (TO 5 PACKAGE WITH A VOLUME OF 0.17 CCJ

Figure 16

AIR LEAK RATE VS "EQUIVALENT”

HOLE DIAMETER FOR FLATPAK

Figure 17

42
equivaleni

mole

diameter

in

microns

WATER VAPOR LEAK RATE AS A FUNCTION OF

"EQUIVALENT'' HOLE DIAMETER IN FLATPAK

Figure 18

AIR LEAK RATE VS WATER VAPOR LEAK RATE

Figure 19

WATER VAPOR PENETRATION TIME AS A FUNCTION

OF AIR LEAK RATE FOR VARIOUS TYPE PACKAGES

Figure 20

43
MOISTURE PENETRATION TIMES

TIME TO EXCEED

VOLUME METHOD

MAX. ALLOW. L.R. <L) (1018) AVG. AMB. - 10,000 PPM

1

I < 0.4 CC

, R = (5 x 10 * CC/SEC) < 0.4 CC

R = (5 X 10 * CC/SEC)

FIXED He

(.01 CC)

FIXED He

(.39 CC)

4.5 X 10 * CC/SEC

2.8 X 10

7 CC/SEC

5000 PPM 500 PPM

71 DAYS 5 DAY8

1.2 YEARS 32 DAYS

Figure 21

MIL-STD-883B; "FIXED"

METHOD, HELIUM FINE

> 0.4 CC FIXED He LEAK TEST. (A!

)

R = (2 X 10

7 CC/SEC) (.4 CC) 6 X 10

7 CC/SEC 214 DAYS 15 DAYS > 0.4 CC FIXED He

R = (2 X 10

7 CC/SEC) (5 CC) 2 x 10 * CC/SEC 2.2 YEARS 57 DAYS

MOISTURE PENETRATION TIMES

TIME TO EXCEED

MAX. ALLOW. L.R. (1018) VOLUME METHOD <L) AVG. AMB. 10,000 PPM

5000 PPM 500 PPM < 0.01 CC FLEXIBLE (He) 5 v 10 * CC/SEC 58 DAYS 4.1 DAYS

(.009 CC)

.01-.4 CC FLEXIBLE (He)

(.01 CC) 1 X 10

7 CC/SEC 32 DAYS 2.3 DAYS

(•4 CC) 1 X 10

7 CC/SEC 3.5 YEARS 92 DAYS > 0.4 CC .41 CC IX 10 ' CC/SEC 132 DAYS 9.4 DAYS

5 CC 1 X 10 • CC/SEC 4.4 YEARS 114 DAYS

Figure 22

MIL- STD- 883B : "FLEXIBLE"

METHOD, HELIUM FINE LEAK

TEST. (A2 )

MOISTURE PENETRATION TIMES

VOLUME METHOD

MAX. ALLOW. L.R. <L)

TIME TO EXCEED

(1018) AVG. AMB. 10,000 PPM

5000 PPM 500 PPM

< 0.01 CC FIXED (K “)

(.009 CC)

1 X 10 * CC/SEC .79 YEARS 21 DAYS

> 0.01 FIXED (K“)

(0.01 CC)

5 X 10 ' CC/SEC 64 DAYS 4.6 DAYS

(0.4 CC) 5 x 10 ' CC/SEC 7.0 YEARS .5 YEAR

(5 CC) 5 X 10 1 CC/SEC 88 YEARS 6.3 YEARS

Figure 23

MIL- STD- 883B ; "FIXED"

METHOD, KRYPTON 85

FINE LEAK TEST (B)

COMPARISON OF THE MAXIMUM ALLOWABLE LEAK RATES ACCORDING TO

MIL- STD- 883B , METHOD 101 A, FINE LEAK TESTS (CONDITION Ai

,

A2 and B) AND THE TIME TO EXCEED PRESENT (5000 ppmv ) AND

FUTURE (500 ppmv ) MAXIMUM ALLOWABLE INTERNAL PACKAGE MOISTURE

LEVELS (METHOD 1018).

44
MOISTURE

CONTENT:

PPM

EFFECTS OF PACKAGE VOLUME

xCM

.17

.37

.79

1.7

3.7

7.9

INTERNAL AREA CM2

.08

.38

1.7

8.2

38

175

INTERNAL VOLUME CM5

.001

.01

.1

1

10

100

RATIO AREA

VOL. Figure 24

80

38

17

8.2

3.8

1.75

MAXIMUM ALLOWABLE PACKAGE

MOISTURE CONTENT

VOLUME

MOISTURE CONTENT

MOISTURE CONTENT

APPARENT

SURFACE AREA WATER

THICKNESS NUMBER

OF MONOLAYERS CM3 PPM, GRAMS CM2 CM -

.001 13,000 9.6 10

* .08 1.2 10

' 3 Figure 25

.01 6.200 4.6 10 * .38 1.2 • 10

' 3

0.1 2,700 2 ' 10

' 1.7 1.2 10

' 3

1.0 1,300 9.8 ' 10

’ 8.2 1.2 10

' 3

10 600 4.6 > 10

* 38 1.2 • 10

'

. 3

100 280 2.1 .< 10

* 175 1.2 • 10

' 3

MAXIMUM ALLOWABLE MOISTURE CONTENT AS A

FUNCTION OF INTERNAL PACKAGE VOLUME

Figure 26

45
MOISTURE INGRESS TIME

TIME CONSTANT

TIME TO REACH ONE(1) T AND 3 MONOLAYERS OF HaO

VOLUME

C.C.

L.fl. ATM CC/SEC

T HOURS

TIME TO REACH

3 MONOLAYERS

HOURS (AVO AMB

13K PPMy)

.001 1 ' 10

' 11 55

.001 1 ' 10

’ 110 550

.001 1 V 10

* 1,100 5.500

.01 1 « 10

* 110 66

.01 1 - 10

7 1.100 660

.01 10

* 11.000 6.600

.1 10

* 1.100 157

.1 10

' 11.000 2.570

.1 10

* 110.000 25.700

1 10

* 11,000 1.283

1 10

' 110.000 12.830

1 10

• 1,100.000 128.300

10 10

* 110.000 5,500

10 10

' 1.100.000 55,000

10 10

* 11.000.000 550.000

100 10 • 1,100.000 27,500 100 10

7 11.000.000 275,000

100

l_

10

* 110,000.000 2.750.000

Figure 28

TIME TO REACH 3 MONOLAYERS OF H20

AS A FUNCTION OF PACKAGE INTERNAL

VOLUME AND AIR LEAK RATE.

(AVO AMBIENT 13.000 PPM,)

46

Figure 29
1.3 THE NID® VAPOR DETECTION TECHNIQUE;

A BREAKTHROUGH IN GROSS LEAK TESTING

by; Bill Evans, WEB Technology, Inc, Dallas, Texas

Leak detection in integrated circuits is a very important high

reliability test which is unfortunately often underemphasized.

Moisture penetration and the subsequent device corrosion is a major cause of field package failures, some of which have

recently been highly publicized. The sources of moisture in

these packages have been described in several papers and can

basically be categorized into the following four groups;

1) Entrapment due to poor bake out procedures and/or sealing

environment

.

2) Generated internally by decomposition, material outgassing or

devitrification of sealing glasses

3) Permeation thru polymer seals

4) Penetration thru cracks and/or faulty seals

It is catagory #4 that this paper will deal with;

Although all packages will leak at some rate, a device can be

described as hermetic provided the leak rate is small enough to

assure the internal atmosphere can be maintained at an acceptable

humidity level for the desired service life of the specimen. Due

to the relatively small internal volumes present within most

IC's, the allowable leak rate constraints are very stringent. It

is generally considered necessary by Mil Standard .requirements to

reject any device with a leak rate greater than 1 X 10-8 atm

cc/sec. Some would argue that this is not stringent enough since

it has been proven that water vapor readily penetrates typical IC

packages with leak rates in this range. However, it is not

feasible to detect leak rates smaller than this value in a production environment by current methods due to surface

porosity, ambient background levels, and equipment utilization.

Luckily, considerable evidence indicates that virtually all leaks

occur at glass-to-metal seals, ceramic-to-metal seals, etc. and

are 5 X 10-7 atm cc/sec or larger. Also, for devices with larger

internal cavity volumes, slightly larger leak rates are allowable

because more moisture is required to reach a concentration level

that causes corrosion.

To facilitate testing over the entire range from 1 X 10-1 to 1 X

10-8 atm cc/sec, it is almost always necessary to split the hermiticity test into at least two catagories; fine leaks and gross

47
leaks. (Sometimes a third category, visual rejects, is also

included). This is due in part to the characteristics of leaks

and which type of flow will occur into the package. This flow

will depend on the relative magnitude of the minimum dimension of

the leak path and the mean free path of the gas molecule. If the

rate at which the gas flows through the leak is governed by wallto-molecule collisions, the flow is considered to be molecular.

This occurs when the mean free path is much greater than the leak

path "diameter". In this flow regime, the flow rate is proportional to the difference between the partial pressures at either

end of the leak and is inversely proportional to the square root

of the molecular weight of the gas passing through the leak path.

If the rate at which the gas flows through the leak is governed

by molecule-to-molecule collisions the flow is considered to be

viscous. This occurs when the mean free path is much smaller

than the leak path "diameter". In this flow regime, the flow

rate is dependent on the viscosity of the gas and the difference

in the pressures at either end of the leak. Between these two

limits there is a gradual change from molecular to viscous flow.

This region is considered the transitional flow regime.

Leakers in the gross leak range (usually considered to be 10-1 to

10-5 atm cc/sec.) typically fall into the viscous and/or transitional flow regime, while leakers in the fine leak range (10-6

to 10-8 atm cc/sec) are almost completely of the molecular flow

type. See Figure 1 below.

Number of Devices

Standard Leak Rate, atm. cc/sec

FIGURE 1

Distribution of leak rates and the flow regimes for a large number and variety of packages. (From reference 1)

48
The most commonly used methods for detecting the smallest leak

rates utilize either a helium mass spectrometer or the Kr85

radioisotope technique. These methods incorporate preconditioning the package with a tracer gas. Due to the molecular flow

characteristics for a fine leak, only gases have a molecular size

and design small enough to penetrate these microscopic leak

paths. If a large enough path is present, during preconditioning this tracer gas will be forced into the cavity. A

reject can then be detected by using either a helium mass

spectrometer or by detecting the gamma radiation being emitted by

a reject with a scintillation crystal and associated circuitry,

depending on the gas used.

In order to prevent rejections of good product caused by surface

adsorption of the tracer gas, it is necessary to have a "dwell"

time to rid the surface of any residual gas. The loss of tracer

gas from the package cavity through the leak path during the

dwell time restricts the largest leak which can be detected. For

a .lcc cavity device and a dwell time of 1 hour (permitted by Mil

Std. 883), the upper detectable limit is about 1 X 10-4 for the

helium test and 3 X 10-4 atm cc/sec. for the radioisotope method.

Because of this, there is a tremendous need for an accurate test

method for detecting leak rates larger than these values and also

showing excellent crossover into the smaller leaks for picking up

the rejects which do not conform to the molecular flow

assumption

.

There are currently many detection techniques used for detecting

these larger leak rates, known as the gross leak range. However,

only a very few are approved by military test standards. These

tests include the fluorocarbon bubble test, weight gain analysis,

dye penetrant inspection, and recently the fluorocarbon vapor

detection technique (a.k.a. NID® Method). Each of these methods

involves a preconditioning of the device(s) prior to actual

testing. This preconditioning in each case deals with attempting

to force a liquid through any available leak path. The presence

of this liquid is then later detected and the device is rejected.

The dye penetrant inspection became popular in the 1960's and was

first used on glass diodes and other devices with glass

envelopes. In effect, a dye is pressurized into the devices and

a 100% inspection is performed. The leakers are identified by

looking for the presence of dye inside the package. As you may

suspect, this method is very limited for production use and has

its largest application when used with non-glass devices as a destructive failure analysis tool.

49
The weight gain test also became accepted during this period.

The idea behind this method is that when an empty device is

preconditioned with a liquid, and a leak path is available, it

will fill with the liquid and gain weight. By taking initial and

final weight measurements on each part, you can determine which

devices leak by the weight increase. This method, when correctly

applied using the proper weight scales, preconditioning sequence,

viscosity fluid, and rejection level, is probably the most accurate method for detecting gross leakers in most semiconductor

packages. However, due to the operator intensive nature of the

test and the fact that it is considerably slower than other

methods, it is much more expensive to perform and is thus not

very popular in high volume applications.

Undeniably, the most widely used method currently for high

reliability Military Standard gross leak testing of semiconductors is the two fluid bubble test. This test method developed

from what is now commonly referred to as the "Old Cl" bubble

test. This original test consisted of submerging the packaged

semiconductor in a bath of fluid which was typically maintained

at a temperature of 125 degrees C. This increase in temperature

caused the air inside the cavity to expand and thus a bubble

would occur as the air escaped, if a leak path was available. It

was soon determined that this method did not consistently detect

the leakers in the 10-4 to 10-5 atm cc/sec range. Because of

this lack of crossover with the fine leak tests, it was realized

that a better test was a necessity. This led to the development

of the "two fluid" test which utilizes two separate fluids with

significantly different boiling points. The package is first

preconditioned with an inert liquid having a relatively low boiling point to force some of this liquid into the internal cavity

through any available leak path. This preconditioning consists

of first subjecting the package to a vacuum environment in an attempt to completely evacuate the cavity of the device. The

vacuum chamber is then backfilled with the low boiling point

liquid, without breaking the vacuum, and pressure is applied to

the chamber to drive the liquid through any large enough leak

path. This pressure is later released and the devices remain

submerged in the liquid bath until just prior to the actual test.

The package is then removed and allowed to dry for a short period

of time before being immersed in the second liquid which is held

at a temperature that far exceeds the boiling point of the

preconditioning liquid. After immersion, the package temperature

50.
is raised which causes vaporization of any preconditioning liquid

in the internal cavity, resulting in a substantial rise in the

internal pressure which is vented through the leak path. This

venting creates bubbles with significantly greater frequency and

duration than the single fluid bubble test. This test in a "lab"

environment, when only testing 1 to 3 parts at a time, can be

shown to be accurate and repeatable down to the 10-5 range and

can detect a substantial number of devices with leaks as small as

10-6 atm cc/sec.

However, there are some distinct draw backs to this technique

when placed in a high production, high stress environment.

Foremost is the dependence on an operator, doing a highly subjective task, under what are less than ideal circumstances. These

bubbles have to be detected by the operator who may have to contend with particles floating in the liquid, spurious bubbles

created by contamination and porous surfaces, bubbles caused by

the release of trapped air during submersion of the tray on which

the devices rest, and eye fatigue. Other factors which can

easily result in poor detectability of leakers lie in the number

of packages which are typically loaded into the bubble tester at

a time, and the way in which they are loaded. For example, some

suppliers are currently testing up to 175 DIPS at a time, stacked

side by side on the tray. Since the largest majority of leaks

occur between the leads on the side of DIP packages (this is the

shortest distance between the cavity and the outside

environment), if the packages are placed side by side and a stream of bubbles are coming up between them, it is very difficult to determine which package is leaking. Also, because the

reject package is generally not removed until after the tray is

taken out of the liquid, even if it was obvious which package was

leaking in the bath, there is a high probability of error in the

operator loosing his/her place and rejecting the wrong part. It

is because of this abuse that the government agencies reportedly

will be applying pressure to decrease the number of packages

which can be tested at once with the bubble method; the number

most often mentioned is 10 devices/test maximum.

Other, often overlooked, factors concern the repetitive nonstimulating nature of this test and the effect on the human

operator. These human effects are generally broken down into

three basic catagories, all of which are interdependent. These

catagories are the job characteristics, the operator

characteristics, and the signal characteristics.

51
The job task is inherently boring which has many negative

implications. It gives an employee no sense of satisfaction because they do not receive feedback about their work, or see how

it contributes significantly to the world. The task demands

nothing of the operator and quickly becomes habitual which allows

the person to direct attention elsewhere. One method of overcoming this boredom is to intentionally increase the rate of rejection (actually elevating the "false alarm" or "overkill" rate at

the company's expense) in order to demonstrate to themselves and

others that their job does serve a useful function. The most unsatisfied employee would likely be one who puts in a full day's

work only to segregate out a minimal number of leakers. At the

end of the month, the work volume typically increases and the

task becomes more stressful. This could actually improve performance in that the work is no longer seen as boring. However, at

this time, the job will also take on the characteristics of a dual task because the operator is required to monitor the tank

and load and unload trays in rapid succession. Because the trays

require active physical involvement (movement of arms to do the

loading and unloading), this portion of the job will receive the

majority of an operator's attention. This activity serves as a needed source of excitement, which has been called response induced stimulation. Again the less bored worker is one who may be

producing at a high rate (processing many ICs) and they would

therefore typically be directing attention to loading and unloading trays instead of observing the tank.

Performance feedback on this task can be related to quality and

quantity of work by an employee. Generally the most notable type

of feedback to the bubble test relates to the number of units

processed, while the quality feedback may generally be nonexistant or delayed over a long period of time which weakens its

impact. The sooner the feedback, the more readily it is attended

to. One form of quality feedback, which is generally at the

operators disposal, can result in what is referred to as probability matching. This is a task related problem encountered in

which the operator knows the number of targets per hour that

should on the average be detected and accordingly reports such

findings regardless of the number of actual targets. This may

increase or decrease both the overkill rate and the escapement

rate merely because an operator knows that he or she should find

only "two leakers per hour"; and that is what they consistently

will do. As time passes, the expectation of detecting a leaker

will increase because an operator is anticipating a leaker to be

52
more probable. When a questionable IC is detected after a long

period of finding no leakers, an operator will be more likely to

throw it out (regardless of its real status) just to satisfy the

quota. The detection threshold then rises because the operator

does not expect another leaker for "half an hour".

On a more abstract level, humans are naturally programmed to do

poorly on the task. From an evolutionary perspective, it is more

"adaptive" to expend energy in challenging or stressful situations and conserve energy in monotonous or boring situations.

This conservation of energy idea is most clearly seen in development of habits in order to avoid useless energy expenditures on

simple everyday tasks. In essence, people are "hard wired" to

habitualize the boring task of detecting leakers so that when a stressful situation arises they will have the energy to attend to

it. Escapement of bad product is associated with neglect or the

misdirection of attention that commonly comes with this boredom.

As it appears in Quality Assurance:

If not enough signals from the outside world are received, the

reticular formation ceases to send the awakening pulses to the

meninges, which then turn off the brain. The person dozes off

and eventually falls asleep. This is an ingenious procedure to

give the brain a pause to relax and recuperate when it is not

needed

.

(1982 June, pp. 37)

With this, we could say that the fault is not with the dozing

operator but instead with the job design. A normal person should

be expected to either sleep or have wandering thoughts to other

more stimulating stimuli. From this, we can predict that initially escapement rates would be low and increase with time as

the individual gets bored. Likewise, the overkill rate is high

initially and decreases with time. Studies have shown this to be

true and defined it in terms of "thresholds of sensitivity". At

the beginning of a vigilance task, the individual is set and

poised to find the leaker. They are expecting it to occur. They

would rather be safe than sorry and reject a greater number of

items, thereby proving to self and boss their value. Their

threshold of rejection is very low, and they will throw out many

good semiconductrs because of their excessive attentiveness to

53
the task. Put another way, they are too attentive to the task.

With time however, attention wanes and escapement becomes the

greater problem. This is the insurmountable problem for the

bubble test. It provides no variation of stimulation for an

individual's five senses. The only support for the test is that

there is some arm movement which keeps the worker somewhat attentive to the task. But even then, escapement may be elevated because this activity is the more stimulating portion of the job.

As mentioned before, this is especially true when time pressures

become critical at the end of the month.

The paradox of vigilance performance in this situation is that

the task demands attention but has a low stimulous payoff. If

there was more of an interaction between the operator and the

task, performance would increase. As previously mentioned,

operators can artifically raise the rejection rate (creating

false alarms), thereby making the job more interesting. This

"false alarm" strategy has been used in the military for radar

operators who have to detect green acoustically-silent "blips" on

a CRT screen. Phantom "blips" are identical to actual signals

but there is no actual object to correspond to its presentation.

It serves only to arouse the operator while also giving performance feedback to upper level personnel who sent the phantom

signal. Incentives have been shown to have little impact on the

quality of detection rates; money, and threats have yielded

little improvement. Only changes in stimulation levels (via

frequent work-breaks, supervisor observation, and random ambient

auditory stimulation) have been shown to improve performance; but

these are costly and interfere with other employees.

One basic operational characteristic finding relates to personality dimensions of introversion and extroversion. The findings are generally that extroverts do better on vigilance tasks

because they direct their attention outwards to the task at hand.

Introverts do worse because they are prone to attend more to

their own personal thoughts at the expense of heightened environmental sensitivity. This finding must be qualified by the fact

that the extroverts are also going to have a greater need to interact and talk with others which would cause a performance

decrement. To summarize, there is no ideal personality type to

run a bubble tester for prolonged periods of time. Persons in

general will do their best in the morning, at about 11:00 A.M.,

which is when most people are at the peek of their ciradian

rhythm. Performance in the afternoon is markedly worse than performance in the morning. Also, the fewer the number of breaks

the worse the performance will be.

54
From a signal characteristics point of view, the overkill and escapement rate are clearly related to the perceptibility of a leaker relative to the other good ICs. This is what is commonly

referred to as figure ground perception. With the bubble test,

there is not much of a figure ground distinction; that is,

"targets" are not readily identifiable because a large number of

bubbles are occuring each time the tray is set in the fluid.

Secondly, the color of the bubbles is not conspicuous relative to

the color of the rest of the tank, which further reduces the discriminatibility of a target. Also, the signal is highly variable

(as defined by the many criteria of defining a leaker) which adds

to the confusion of determining whether a device is a reject or

not

.

Each of the findings presented above may be said to have at least

some impact on the vigilance task relating to detecting leakers.

Taken as a whole, it is clear that there may be multiple causes

for the poor performance records found on this task. The factors

are time and stimulous related, which make the job exceedingly

difficult to adapt to human weaknesses. This problem is best

handled by letting a machine detect the rejects.

Other negative aspects of the bubble test in production involve

problems associated with material flow through the process steps.

Because the bubble test is inherently a batch test and involves

operator intervention, there has been little to no success in

automating this technique. Also because of this batch loading,

many manufacturers insist upon doing 100% testing prior to lead

trim operations to help avoid bent lead problems. This results

in only a sample test after trim which is undesirable since the

trimming operation is known to be a significant cause of both

fine and gross leaks, especially for cer-dip devices.

THE NID GROSS LEAK DETECTION TECHNIQUE OVERCOMES VIRTUALLY ALL OF

THE PROBLEMS ASSOCIATED WITH THE BUBBLE TEST AND IS WELL SUITED

FOR THE HIGH VOLUME PRODUCTION ENVIRONMENT. THIS METHOD:

1) Removes operator dependence and subjectivity.

2) Is automatable and thus allows gross leak testing to be performed end-of-line (where it should be) without causing bent

leads

3) Is very accurate in detecting leaks in the gross leak range.

4) Has very good crossover with both fine leak tests currently

in use.

5) Has certain operating cost improvements.

55
This method utilizes a vapor detection technique. The packages

are preconditioned in a manner similar to the 2 fluid bubble

test; however vacuum and pressure times are reduced due to the

improved sensitivity of the technique. After removal from the

preconditioning fluid, the package to be tested is placed, either

manually or automatically, into a test cell heated well above the

boiling point of the preconditioning liquid in such a manner as

to achieve good thermal contact. This causes the package temperature to elevate and thus an internal vaporization of the liquid occurs. Much like the bubble test, these vapors build up an

internal pressure and an outgassing through the leak path will

occur. The air around the device is passed through a proprietary

detector and is sensed for the presence of these vapors. This

detector is designed for maximum iccuracy with a minimum of service related problems. It features a very low zero drift, is

repeatable over long periods of time, is not flow sensitive, and

the gas sample is isolated from the actual detector thus avoiding

contamination. Another very important design criteria is that,

in the event an equipment failure does occur, virtually anything

that happens will result in the detector going "off scale" insuring a failsafe condition.

The sensitivity of the test is a function of the dilution volume,

the integrating time, and the lower detectable limit of the

analyzer relative to any leak rate. This dilution volume is the

volume of air contained in the test cell and all connecting

tubing between this test cell and the detector when the unit is

integrating the gas concentration (this is commonly referred to

as the measure or test mode). The leak rate, or outgassing, of a device is a function of the fluorocarbon boiling point, the temperature (and corresponding vapor pressure) inside the cavity,

the geometry of the leak path(s), and the preconditioning

conditions

.

It can be shown mathamatically that a 10-5 atm cc/sec leaker

would leak at the rate of 10 ppm into a lcc volume at standard

conditions. As the temperature is increased, this flow will increase as the square of the resulting pressure increase, assuming

the leak rate is in the viscous flow range. Since the lower

detectable limit of the NID approaches 10 ppb with a lcc volume

dilution, the sensitivity is sufficient to detect leakers well

below the 10-5 atm cc/sec level thus assuring good fine leak

crossover. Also, since the device is outgassing into a vapor

56
environment/ the NID method should be more sensitive than the

bubble test because the internal pressure of the device does not

have to overcome any detector liquid's static pressure to achieve

outgassing

.

To go from a laboratory detection technique to an automatic high

volume production test, with the ability to accurately test one

part at a time, required determining measuring time parameters,

acceptable dilution volumes, preconditioning times, etc. This

was done and reported in SME report #IQ84-679 presented at the

Qualtest 3 conference in October of 1984. Based on the test data

on the NID, the production parameters of a 3 1/2 second measure

cycle after preheat in 125 degree C heated test cell and a reject

level of 200 were shown to be adequate to crossover with the fine

leak test. To increase this overlap, features were added to the

computer controlled automatic NID. These include a P.A.L. (pre

alarm level) reject and also a software extrapolation mode

reject. These features essentially detect the presence of escaping vapors. If the vapor concentration does not achieve the

reject level, but is high enough or would become high enough

given additional testing time to warrant further investigation,

the software can be directed to either reject the device or put

it in a special output tube. Based on correlation studies at

numerous locations and parallel screens, the system as designed

has demonstrated that a bad part will be detected by either the

NID or by the fine leak test(s). Besides the measurement

parameters, the design had to also accommodate actual production

conditions. For example, the fluorocarbon background at various

manufacturing facilities has been found to vary considerably. If

the NID sensitivity were increased above current levels, it would

make the instrument more sensitive to these background levels and

could result in good parts being rejected due to fluctuations in

the fluorocarbon concentration of the atmosphere surrounding the

test cell. This concentration can increase substantially as a result of nearby venting of preconditioning tanks, spillage of

fluorocarbon, or several other common production occurances.

In as much as the sensitivity data was based on perfect conditions (i.e., a single, straight, round hole), one can not do an

exact one to one correlation. Hence, adequate overlap must be

demonstrated in production testing. In several tests involving

groups of fine leak rejects, the NID method was found to detect

up to 70% of the samples. The explanation for this excellent

crossover is due to the NID system relying on opposite parameters

57
than the fine leak tests. Specifically if a part is a large

leaker in the viscous flow range, the fluorocarbon outgassing

will occur at a significant rate to be easily detected by the

NID, whereas the helium or krypton gas would have escaped prior

to measurement. If, on the other hand, a fine leaker in the

molecular flow range is tested, the opposite will be true; the

helium or krypton will remain to be detected and the hole size

will be too small to either allow liquid fluorocarbon in and/or

adequate fluorocarbon outgassing to achieve a NID failure (200

count threshold). Further analysis of samples which did outgas

readings greater than zero but less than 200 have been repeatably

shown to be either fine or false leakers. These parts will typically have a zero or minimal weight gain, may show a short burst

of bubbles in the bubble test and fail fine leak. It appears

from x-rays that many times the parts have a ceramic or seal

porosity problem and may or may not actually have a leak path to

the cavity; if there is a leak path it is of the fine leak

variety

.

This method has shown excellent correlation with the weight gain,

dye penetrant, and bubble test procedures in one on one correlations under laboratory conditions. In similar tests with the 2

fluid bubble test in a production environment, the NID method has

proven to greatly reduce overkill and escapements (as predicted

by the preceding paragraphs dealing with the human effects and

job design of the bubble test). See Tables 1 & 2. These tables

dramatically point out the need for a nonsub jective gross leak

hermiticity test technique. These samples were all supplied by

major I.C. manufacturers as either random samples from the gross

leak reject "bin" or a random sample of "good" outgoing product.

It should also be noted that these are typical results, not worst

case

.

On top of the quality improvements realized when testing with the

NID, several operating cost improvements are obtained. The most

apparent of these savings is the reduction in the fluorocarbon

usage over the bubble test. With the NID, there is no need for

the high temperature fluorocarbon liquid bath. This liquid costs

in the neighborhood of $ 400 per gallon and must constantly be

replenished due to "drag out" and evaporation. Condensing coils

on top of the bubble tank can help minimize the evaporation out

the top of the tank, however most losses occur from the evaporation of the liquid from the surfaces of the devices and loading

tray once removed from the tank. This saving is generally estimated at 1 to 5 cents/part for standard DIPS and is dependant

on the surface area of the package. Another cost saving which is

realized deals with the reduction in any bent or scratched leads

58
TABLE #1

OVERKILL CORRELATION TESTS

TEST #

DEVICE

TYPE

NUMBER PARTS

TESTED (1)

NUMBER

GOOD

NUMBER

FINE

NUMBER

GROSS % OVERKILL (2)

1 DIP 425 400 10 15 94.1 %

2 DIP 100 45 0 55 45.0

3 DIP 50 4 3 43 8.0

4 DIP 100 35 0 65 35.0

5 DIP 108 81 0 27 75.0

6 DIP 100 96 0 4 96.0

7 DIP 54 5 0 49 9.3

8 LCC 102 14 0 88 13.7

9 DIP 4000 1602 N/A 2398 40.0

10 DIP 228 196 4 28 86.0

11 Flatpack 77 25 0 52 32.5

DEVICE

ESCAPEMENT

NUMBER PARTS

TABLE #2

’ CORRELATION TESTS

NUMBER NUMBER NUMBER % BAD PRODUCT

TEST # TYPE TESTED (3) GOOD FINE GROSS OUTGOING (4)

1 DIP 598 584 0 14 2.3 %

2 DIP 1000 998 0 2 .2

3 DIP 600 592 0 8 1.3

(1) Parts failed bubble test

(2) Number good divided by total number parts tested

(3) Parts passed bubble test

(4) Number gross divided by total number parts tested

59
resulting from the batch preconditioning and testing of devices

in the bubble test. These savings are difficult to put a price

tag on unless you have a very good grasp of your rework costs or

until you have been faced with a return by a major customer.

With the current quality thrust in the industry, a return of

product for a reason such as this is not only embarrassing and

very costly from a monetary point of view, but more importantly,

affects the reputation of your company.

Sometimes, a non apparent cost savings results from the ability

with the NID to do gross leak testing end-of-line where it

belongs and to eliminate most or all non-necessary sample testing

steps. This test technique, because it is automatable, can

greatly improve and streamline the product test flows of assembled packages.

In summary, this new technique is based on the utilization of an

existing, proven technology. Its introduction has finally made

this reliability test step more accurate, less costly, and

automatable. Because of these advantages over the current most

popular methods, measures can now be taken which will greatly improve the outgoing hermiticity quality level of Military Standard

tested product, namely taking steps to do away with the bubble

test for most high reliability applications.

60
REFERENCES

I

Robert Merrett, Test Methods and Their Limitations, 1S84

International Reliability Physics Symposium Hermeticity

Tutorial, 1-2.2 through 1-2.35, April 1984.

Aaron DerMarderosian , Permissible Leak Rates & Moisutre

Ingress, 1984 International Reliability Physics

Symposium Hermeticity Tutorial, 1-3.3 through 1-3.14,

April 1984.

Ralph McCullough , Introduction/History , 1984 International

Reliability Physics Symposium Hermeticity Tutorial, 1-1.1

through 1-1.3, April 1984.

W.E. Briggs & S.G. Burnett, Helium Mass Spectrometer Leak

Testing Of Pressure - "Bombed" Sealed Enclosures, Varian

Vacuum Division, 8, October 1, 1968.

S.B. Banks, R . E . McCullough & E.G. Roberts, Investigation of

Microcircuit Seal Testing, Air Force Systems Command, Rome

Air Development Center, Tech Rep RADC-TR-75-89 , April 1975.

Chuck Wilkinson, Leak Testing Microelectronic Devices In

Accordance With MIL STD 883/ Microelectronic Manufacturing

& Testing, 16-17, May 1985.

Richard Cole, Human Error and Vigilance in Relation to the Bubble

Test, July 1986.

Edward Etess, Development of the NID system for performing gross

leak testing in compliance with Military Standard 883/ Qualtest 3 conference, tech report 1Q84-679, October 1984.

61
1.4 Reusable Moisture Standard Development for Package Gas Analysis*

Ray F. Haack

Jet Propulsion Laboratory

California Institute of Technology

MS 125-112

4800 Oak Grove Drive

Pasadena, CA 91109

(818) 354-6568

ABSTRACT

Initial design and testing of a device for application as a moisture standard

for IC package gas analysis by mass spectrometry was reported at the RADC/NBS

Moisture Workshop III. This device can be assembled at the analysis site and

analyzed concurrently with IC packages. After analysis, the device can be

disassembled and the sealing disc replaced; thus the package can be reused.

Several packages have been sealed at ambient temperature in an atmosphere of

either air or nitrogen with a known moisture content. A constant humidifer and

optical dew-point hygrometer connected to a glove box provided the sealing

environment. Following helium leak test certification, the packages were analyzed for moisture content by participating laboratories using mass spectrometric techniques. The study involved the use of packages consisting of

vacuum melt stainless steel cavity material, a 16-mil gold-flashed aluminum

sealing disc, and a threaded brass cap for applying the proper torque and

pressure on the sealing disc. The effects of using air and nitrogen as the

sealant gas will be presented and their correlation discussed.

INTRODUCTION

Further improvements in moisture standard packages are needed for attaining a high level of credibility in the electronic package gas analysis via the mass spectrometer (MIL STD 883B, method 1018.2). As specifications decrease the

maximum allowable moisture level, the role of the standard becomes even more

important. In an effort to meet this need, the concept of a reusable moisture

standard package was investigated. The initial package design and test results

were reported at the previous workshop!

.

The review and evaluation of the standard package design resulted in a package

consisting of: 1) 304 vacuum melt stainless steel body with a cavity volume of

-0.05 cc, 2) gold-flashed 16 mil aluminum sealing disc and, 3) threaded brass

cap. Correspondingly, these specific materials were selected to provide: 1)

uniform cavity surface, 2) seal surface, capabil ity and moisture inertness and,

3) torque ease and reusability (anti-galling of threaded surfaces with brass

and stainless steel )

.

*This paper represents one phase of research performed by the Jet Propulsion

Laboratory, California Institute of Technology, sponsored by the National

Aeronautics and Space Administration, Contract NAS7-91B.

62
EXPERIMENTAL

The sealing surface of the stainless steel body was polished (1 micron alumina

powder on felt); then cleaned ul trasonically. The three parts of the package

unit were cleaned, rinsed with methyl alcohol, and air dried at 110°C for 16

hours. The dry parts were transferred to a dessicator for cooling and then to

the glove box for assembly and sealing in an atmosphere of nitrogen or air at room temperature (25 + 1°C) as outlined in Figure 1. The humidifier^ and

hygrometer provided moisture control of the environment during the sealing

operation. The package components and glove box were equilibrated for one hour near the desired dew-point before beginning assembly. An additional one hour

was required to assemble and torque (100 in-lbs) the packages in each series.

Components of the package were handled with forceps. Continous monitoring of

the sealing chamber exit gas showed a dew-point variation of +0.3°C during the

operation.

Sealed packages were subjected to hermeticity testing (15 psig for 12-16

hours). A helium leak rate of > 1 x 10" 8 atm‘cm3/s was initially used as the

criteria for rejecting units. Gas analysis for helium indicated that a leak

rate of < 0.2 x 10' 8 atm*cm8/s provided better assurance that the standard was hermetic. Laboratories were requested to air or vacuum bake the samples at

100°C for a period of 12-16 hours prior to the puncture and gas analysis.

However, the packages were not analyzed in an identical manner. Laboratory A

used a batch system with a 12-16 hour vacuum bakeout at 100°C, Laboratory B used the rapid cycle method at 100°C, and Laboratory C used the rapid cycle

method at 100°C preceeded with an air oven bakeout at 100°C for 12-16 hours.

RESULTS AND DISCUSSION

It must be emphasized that the primary task objective was to seal units at a given time and to establish if consistent results for these packages could be

obtained. Fabrication and process control factors such as materials uniformity

and treatment, hermeticity, and the sealing environment were maintained

constant. The quality of the packages submitted for analysis should be

assessed by the following;

1. relative burst pressure at puncture,

2. presence of argon when air was the sealant gas or, lack of argon when

nitrogen was the sealant gas,

3. minor constituent gas anomolies,

4. percent moisture (volume/volume)*.

This study consisted of three series of experiments using the same reusable

packages. Sealing discs were replaced after each use. The packages were

sealed in an air atmosphere (-6000 ppm moisture) for series 1 and 2. For

series 3, the packages were sealed in a nitrogen atmosphere (-6000 ppm

moisture)

.

*Note: The percent moisture should represent that of the sealant gas as monitored by the dew-point hygrometer and the adsorbed surface moisture from

the package cavity and seal disc.

63
Results for the three series of packages as analyzed by three different

laboratories are shown in Table I. Graphical representation of relative burst

pressure, percent argon, and percent moisture appears in Figures 2, 3, and 4

(the package designation corresponds to the order sequence in Table I).

The interval between seal date and analysis date varied as follows: Series 1,

20-60 days; Series 2, 29-48 days; Series 3, 13-14 days.

Series 1. For seven of the nine packages, the water content ranged from 0.49

to 1.08 with an average of 0.75%. Two other packages were outliers with 0.17%

(S/N 52) and 3.99% (S/N 51). The argon percentages (Table I, Figures 2, 3, 4) are relatively consistent for the packages sealed in an air atmosphere and

analyzed by a particular laboratory. The trace amounts of carbon dioxide

(-0.1%) should be expected since the sealant gas was breathing air. Carbon

dioxide is customarily a part of the gas analysis for electronic packages. In that case, its abundance is important to the understanding of chemical and/or

physical changes which may have occured within the package.

The burst pressures may be subject to the method of analysis (either batch

system or rapid cycle puncture device), the package cavity size, sealing

conditions, and quality of the hermetic seal. The batch system probably

produces the more accurate burst pressure since it represents a static state

whereas in the rapid-cycle system the pressure depends dynamically upon the

size of the puncture hole produced. Batch system measurements (capacitance

manometer, readout of +0.001 Torr) and corresponding package cavity volumes are shown in Figure 5. The close grouping of packages of a given size should be

expected when they are hermetically sealed under similar conditions of

temperature and barometric pressure.

Series 2. For eight of the eleven packages, the water content ranged from 0.40

to 1.0.7 with an average of 0.66%. Two other packages (S/N's 51 and 55) showed

a trace of helium and therefore cannot be considered hermetic. Serial Number

43 (5.79% water content) was an outlier. The percentage of argon found in the

packages (S/N's 42, 46, 52) was high (the nominal value for argon in air is

0.93%). Since these packages were analyzed by the same laboratory, the

determined argon sensitivity factor for the mass spectrometer at that time

could be in error.

Two factors were different for Series 1 and 2. The measured helium leak rates

for the packages in Series 1 were < 0.2 x 10“** atm’cnrys; whereas the rate for

Series 2 packages was -1 x 10“ 8 atm'cm^/s. The second factor was a replacement

of gloves for the glove box before beginning the Series 2 experiment. Although

there does not appear to be correlation with these factors and this data,

earlier work showed that traces Of helium were detected in the packages having

a helium leak rate in excess of 1 x

10"** atm*cnr*/s.

Series 3. At this point, these packages are not considered as a valid moisture

standard test. For this series, only S/N 49 can be considered acceptable. The

other packages must be rejected because of measurable quantities of helium,

argon, and oxygen. Several irregularities are apparent for the nitrogen-filled

packages. Detectable amounts of hydrogen were found (seven out of 10);

coincidentally these packages contained seal discs gold-flashed by a different

vendor (Figure 7). The presence of oxygen could be due to either the sealant

gas quality and/or a leak in the seal. It is not likely that the sealant gas

64
contained oxygen since the analyses do not show a correlation with the sealing

sequence (all packages were filled in sequence within a one hour uninterrupted

time interval). The results for package S/N 48 (presence of air consi ti tuents)

probably indicate an invalid sample; also it was a leaker in both Series 1 and 2.

The moisture content distribution exhibited by the samples submitted for

analysis is shown in Figure 6. As discussed in the previous section (Series 1,

2, 3), some of the samples cannot be considered as valid standard package

tests. Thermal cycling effects may have accounted for the random distribution

of results obtained by Laboratory C. The laboratories which performed the

analyses did not use the same type of sample introduction system although all

analyses were performed at 100°C (method requirement). A random selection of

samples for each laboratory was made so as to minimize any effect from the

sequence of package fill and sealing. If the laboratory used the rapid cycle

method, it was requested that the sample be oven heated at 100°C for 12-16

hours. This may not have been a wise decision considering that the package was thermally cycled twice. With the batch system, each package is thermally

cycled only once. One would expect the total moisture to be in excess of 0.6%

since it includes moisture in the sealant gas and the surface-adsorbed

moisture. The later could conceivably add as much as 50% although the

preparation and handling of the parts precludes it being more (seal hermeticity

excepting)

.

Significant correlation exists between package hermiticity and the vendors

providing "gold flashing" of the sealing discs. Gas analysis of packages

sealed with discs from Vendor B (Table 1) showed the presence of helium, oxygen

and argon and a potential chemical problem (the presence of hydrogen). The

limited sample base precludes any postulation as to the source of hydrogen.

The differences in hermiticity cannot be accounted for on the basis of the

sealing atmosphere. Scanning electron micrographs revealed dramatic

differences for the type of gold-flashed seal discs used (S/N 49 and S/N 56).

Extensive penetration of the gold flash into the aluminum was shown by SEM

using the backscattered detection mode, 100X (Figure 7b). The normal detection

mode, 100X, (Figure 7d) also indicates potential leakage paths which would

allow a loss of hermeticity. The depth of the gold flashing on the discs from

each vendor was also compared (Figure 7e, 7f). The gold thickness for S/N 49

(Vendor A) was 2-3 microns, and that for S/N 56 was 4-5 microns. It should be

noted that further conclusions must include the fact that the other components

of the packages (particularly the seal edge) were not necessarily identical.

CONCLUSION

One important role for a moisture standard package is to calibrate the

sensitivity of the mass spectrometer, particularly in the 2,000 to 5,000 ppm

range which includes the maximum level allowed by MIL STD 883B, Method 1018.2.

The use of this standard would prevent packages with very high moisture content

from escaping detection. The air-filled package has merit since it can be

easily assembled using ambient air. From the analytical chemistry view point,

the nitrogen-filled package can be more definitive, i.e., the hermetic

integrity can be readily determined from the presence of helium, argon, or oxygen. Also, the nitrogen-filled package has a practical merit in that the

major percentage of semiconductor packages are currently sealed in a nitrogen

65
atmosphere. This package design is readily adapted for various cavity volumes;

its ease of assembly makes it possible to prepare standards with various levels

of moisture.

ACKNOWLEDGEMENT

The package hermeticity testing was performed by Kenneth C. Evans; SEM analysis

by Ronald T. Ruiz. The author also wishes to acknowledge the review assistance

from Dr. Alex Shumka (Electronic Parts Failure Analysis Group) and from Sheryl

Bergstrom (Analytical Chemistry Group).

REFERENCES

1. Haack, Ray F., "A Moisture Standard for IC Package Gas Analysis/ RADC/NBS

Workshop Moisture Measurement and Control for Semiconductor Devices, III,

NBSIR 84-2852, May, 1984.

2. Weaver, E.R., Hughes, E.E., and Diniak, A.W., "Determination of Water

Vapor from the Change in Electrical Resistance of a Hydroscopic Film,"

Journal of Research of the National Bureau of Standards, Vol . 60, No. 5,

May 1958, pp. 489-508.

66
LIST OF TITLES AND ORDER

Figure 1. Package Sealing in Controlled Environment

Table I. Package Gas Analysis

Figure 2. Laboratory A Gas Analysis

Figure 3. Laboratory B Gas Analysis

Figure 4. Laboratory C Gas Analysis

Figure 5. Measured Burst Pressures

Figure 6. Moisture Content Distribution for Respective Laboratories

Figure 7. SEM - Gold-flashed Sealing Discs

67
NITROGEN

Figure 1 . Package Sealing in Controlled Environment

PACKAGE DESIGNATION

Figure 2. Laboratory A Gas Analysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 PACKAGE DESIGNATION

Figure 3. Laboratory B Gas Analysis

1 2 34 5 67 8 9 10 11 12 13 14 15 PACKAGE DESIGNATION

Figure 4. Laboratory C Gas Analysis

68
TABLE I. Package Gas Analysis

S/N

Relative | Burst * PressureCOMPONENTS (% volume/volume)

He A r h

2 o n

2 °2 co 2 h

2

Series 1 42 0.98 NA 2 1.00 0.62 85.9 12.4 0.12 NA

45 0.96 NA 1.03 0.49 84.5 13.9 0.12 NA

46 1.04 NA 1.03 0.83 84.9 13.1 0.15 NA

47 0.98 NA 0.99 0.57 80.1 18.2 0.13 NA

54 1.04 NA 0.97 0.76 79.8 18.5 0.15 NA

55 1.01 NA 0.99 0.87 82.8 15.2 0.16 NA

43 1.08 ND 3 0.90 1.08 77.3 20.5 0.16 ND

49 0.97 4 ND 0.88 0.59 78.5 19.8 0.18 ND

52 0.95 4 ND 0.96 0.17 77.6 21.2 0.08 ND

44 1.08 0.013 0.74 0.98 79.0 18.8 0.04 0.00

48 0.00 NA

51 0.92 0.00 0.72 3.99 77.7 17.3 0.02 0.00

Series 2 42 0.99 NA 1.36 0.43 83.2 15.0 ND NA

46 1.04 NA 1.46 1.07 83.2 14.3 <0.3 NA

48 0.00 NA

52 0.98 NA 1.29 0.78 81.5 16.4 <0.3 NA

44 1.15 ND 1.11 0.71 76.0 22.2 0.02 ND

45 0.93 ND 1.08 0.40 77.0 21.4 0.09 0.01

49 0.86 ND 1.09 0.39 76.8 21.7 0.06 0.03

54 1.07 ND 1.20 0.44 76.1 22.2 0.05 <0.01

43 0.98 <0.01 0.83 5.79 75.3 17.6 0.02 0.00

47 1.05 <0.01 0.61 1.07 81.0 17.2 <0.01 0.00

51 0.99 0.01 0.95 1.57 78.3 18.7 0.01 0.00

55 0.98 0.01 0.92 3.85 77.0 17.7 0.02 0.00

Series 3 44* 1.22 2.89 0.03 0.36 95.7 0.99 0.01 0.04

45 0.98 ND 0.03 0.80 98.4 0.75 0.01 ND

49 0.85 ND ND 0.95 99.0 ND <0.01 ND

52* 0.98 0.68 ND 0.83 98.2 0.32 <0.01 0.01

56* 0.98 1.42 ND 0.89 97.3 0.37 <0.01 0.04

43* 0.91 1.22 0.01 4.48 85.8 5.35 <0.01 3.15

48* 1.05 <0.01* 0.75 0.70 79.8 18.7 0.01 0.00

51 0.90 0.01 0.00 3.77 96.2 <0,01 0.00 0.01

54* 0.95 <0.01 0.00 63 35.0 0.19 0.00 1.76

57 5 1.20 0.09 <0.01 2.64 94.3 0.38 0.00 2.55

J Calculated with average of each analysis group as reference.

2 Not Available.

3 None Detected.

4 Seal disc decreased in thickness at time of analysis.

5 Machined cavity depth of 0.145"; other packages 0.110 i 0.005"

* Gold flashing by Vendor B.
BURST

PRESSURE

(torr)

Figure 5. Measured Burst Pressures 70
o

o

DC

m

on

LU

DC

I I I IT)

DC

c

I CNJ IT) LU

DC

LUm

DC

<

on

LU

DC

LUon

CC

LU

QO

<

DC

LUon

LUO

<co

<

D_

(3WmOA/3WniOA%) 3 dfllS I0W

71

Figure

6.

Moisture

Content

Distribution

for

Respective

Laboratories
SEAL D

I

BACKSCA

Figure 7. SEM - Gold-flashed Sealing Discs

72

SEPEKM KTTT W
2.1 MOISTURE PROCESS CONTROL AND MASS SPECTROMETER CORRELATIONS

R . K . LOWRY

Harris Semiconductor Analytical Services Department

P.O. Box 883, M/S 62-07

Melbourne, Fla. 32901

305/724-7566

I . Introduction

Technologies for controlling internal water vapor content

of hermetically sealed IC packages have matured greatly over

1-3

the past decade. In fact, for semiconductor packages

which do not contain organic materials, achieving low internal

moisture at seal is primarily a matter of conscientious and

consistent process execution.

The MIL-SPEC internal water vapor limit has evolved as a generic specification for the IC industry. Users of IC

products have come to expect that hermetically sealed

packages, whether "hi-rel" for aerospace/military applications

or "commercial" for less stringent uses, will contain less

than 5000ppmv of moisture sealed in the cavity.

For this reason extensive moisture control precautions

are common practice in most manuf acturers 1 assembly sealing

operations. This paper discusses a body of data from an

overseas assembly facility for commercial IC products being

sealed into TO-99 header-style packages.

I I . TO Package Assembly Engineering

A. Package Construction

The TO-99 package consists of a gold plated metal

substrate with hard glass feed throughs for the pins and a nickel cap. The cap is welded onto the header near room

73
temperature in a controlled N

2 atmosphere to form the

package seal. The internal volume of the sealed cavity is

typically 0.05cc. This is a batch operation in which 200-2000

packages are sealed in a group in the welder chamber, and then

removed to allow another large group of piece parts to be

placed in the chamber for sealing.

Engineering studies over the years at the overseas

assembly location where this process is run revealed a number

of precautions necessary for controlling internal water vapor

content of TO-99 cans.

B . Moisture Control in TO Packages

With the advent of the TO package years ago, it was

thought that the principal requirement for product dryness was

keeping moisture low in the sealing chamber. Welders have

always contained sensors for monitoring "drybox" moisture.

But experience has shown that low welder moisture levels are a necessary but far from sufficient condition to assure package

dryness

.

The piece part materials themselves are the principal

source of moisture in TO cans. The seal glass and the metal

parts contain both chemisorbed surface water and bulk water

which can off-gas after seal to contaminate the package

cavity. An examination of the condition of the sealing glass

at 30X will reveal its potential for moisture volatilazation.

The parts can exhibit blowholes and/or meniscus cracking of

the glass, providing escape paths for internally trapped water

to the cavity.

74
Vacuum baking is imperative to dependably desiccate the

package piece parts. Both headers and caps are loaded into a vacuum chamber directly adjacent to the sealing chamber. They

are vacuum baked well below 100 mtorr pressure. Pumpdown time

to working pressure is in addition to the low pressure residence time. When the vacuum bake is concluded, the parts are

transferred to the sealing chamber with minimal exposure to

room ambient. The chamber contains appropriate moisture

sensors and its ambient is ultra-dry N

2

at atmospheric

pressure

.

III. Mass Spectrometry and SPC for Moisture Control

Process optimizations described above were reached as a result of extensive engineering studies. All the moisture

measurements for the studies were made by mass spectrometry.

As an outgrowth of the studies, a plan evolved for the ongoing assurance of moisture control in the TO sealing process. The plan is based on the principles of statistical

process control (SPC). The SPC approach verifies the quality

of a process based on sampling for attributes, in this case

package moisture content. It helps maintain a consistent mean

for the process with a continual monitor of the process'

natural variability. An SPC approach to moisture control in

_ 4 Cerdips has previously been reported.

In the SPC plan, two assembled cans are randomly pulled

from each sealing lot. These are logged along with all

75
pertinent information regarding the sealing of that lot. All

samples of each day's production are accumulated and forwarded

to the analytical laboratory. The laboratory analyzes one

sealing lot per day each of eutectically mounted product (two

specimens) and epoxy mounted product (two specimens) by mass

spectrometry. The mean moisture level and variability are

monitored by SPC charts maintained separately on the eutectic

and epoxy mount processes. Any reading above the Upper

Control Limit (UCL) for the process being monitored causes

samples of the lots sealed just before and just after the

out-of -control lot to be immediately pulled and analyzed.

In this way, out-of -control conditions are quickly

identified and time-bracketed while an assignable cause for

the out-of -control condition is found. This also provides a means of isolating any product lots which may exceed

engineering tolerances on internal water vapor.

Process control data governing the TO sealing as well as

other hermetic package sealing processes have been measured

for a number of years in the Harris analytical services mass

spectrometer. This is a Pernicka Corporation rapid cycle

instrument with three volume calibration valve providing

calibration bursts of 0.01, 0.1 and 0.8cc. This instrument

was certified by DESC in 1985 for measurements pertinent to

the 5000ppmv MIL SPEC limit after a series of calibration and

round robin analysis experiments. The SPC data discussed here

span the time before, during, and after all the certification

activities

.

76
IV. Process Baseline Statistical Data

A . Eutectic Process

Fig 1 is the histogram for 362 data points which

characterize the eutectic sealing process for a one year time

period approximating CY 1985. The data is normally

distributed

.

Figs 2A and 2B are the control charts governing the

process. The process mean is 1527ppmv f with a three sigma

Upper Control Limit of 2623ppmv (as calculated from the R

value )

.

B . Epoxy Die Attach Process

Though carried out in the same process equipment, the

epoxy process is charted and controlled entirely separately

from the eutectic process because the presence of epoxy in the

sealed headers adds a new technical dimension to controlling

the moisture attribute.

Fig 3 is the histogram for 173 data points which

characterize the epoxy sealing process for the same time

period as the above eutectic data. The data is approximately normally distributed, with a bit of tail-off in the

high end of the range.

Figs 4A and 4B are the xR charts for the epoxy process.

The process operated at a mean of 2267ppmv with a UCL of

454 lppmv.

C . State-of-t he -Process

Table IA summarizes the state of the header sealing

process for both eutectic and epoxy processes, during

approximately CY 1985. 77
PROCESS

MEAN

UPPER CONTROL

LIMIT

TABLE IA*

Eutectic 1527ppmv 2623ppmv

Epoxy 2267ppmv 4541ppmv

^Individual mass spectrometer readings are significant only to

the nearest lOOppmv. All values in this paper are the result

of statistical treatment of sets of data, and thus carry more

figures than are significant to help comparisons of means and

control limits.

These numbers describe the fundamental capability of the

process through CY 1985. The process appears to be operating

at its optimum using approaches described in the previous

sections. No samples of eutectic exceeded 5000ppmv while

three of the epoxies exceeded 5000ppmv. Both processes had

means and UCL ' s below the MIL-SPEC moisture limit. In every

case, where a lot sample exceeded the UCL. the lots before and

after it were analyzed and found to be in control. And in

most cases, lots measured as out of control were found to have

an. assignable cause for the excess moisture, e.g.

non-hermet ici ty of the analyzed sample, process log entries

indicating inadequate bake cycles, etc.

D. Concern for the Process

With the attributes of Table IA, the header sealing

operation was considered to be providing product from a controlled process. But concern began to build that if/when the

MIL-SPEC engineering limit for moisture were lowered from

5000ppmv to 3000ppmv or 2000ppmv the majority of the parts

containing epoxy as well as a non-trivial number of the eutectic parts would not meet the specification, even though they

were coming from a process operating within statistical

control

.

78
While pondering this concern, but before any actions

could be formulated to address the matter, the mass spectrometer became inoperable. It went out of service for more than

2 months

.

Production continued, however, and provisions of the SPC

plan called for maintaining all elements of the plan by

obtaining mass spectrometer data from a certified commercial

laboratory. Conseguently sampling and mass spec analysis

continued uninterrupted, and data began to flow in from the

commercial lab.

V. Statistical Definition of the Process By a Different Mass

Spectrometer

As the data came in, a different picture of the process

started to emerge. Though the sample size is much smaller

than the above data set, the data led to a much different

perception of moisture level in the process.

A. Eutectic Process

Fig 5 is a histogram of the 38 samples of the eutectic

process measured at the commercial laboratory. While there

are insufficient samples to judge whether the data is normally

distributed, the histogram still gives a good picture of how

this mass spectrometer measures moisture in the process.

Figs 6A and 6B are the xR charts for these data points.

Thirty eight points are sufficient for valid control charts

with process attributes calculated as described previously.

79
The process' mean moisture content is now defined as 504ppmv,

with a UCL of 1602ppmv!

When analyzed in this mass spectrometer, the process

attributes are very different. So different, in fact, that

even if the MIL-SPEC moisture limit were reduced to 2000ppmv,

all eutectic product delivered from within the process control

limit would easily meet that specification. We thus get a very different impression of the process from this second

analytical instrument.

B . Epoxy Process

Figs 7A and 7B are the xR charts for the epoxy process.

Sixteen data points are not sufficient for valid control

charts, so the process mean of 774ppmv and the UCL of 2967ppmv

must be considered trial values. Nevertheless, it appears

that a significant downward shift in the process attribute has

occurred just as in the eutectic process.

C. The "Revised" Sta te-of -the-Process

Table IB summarizes the newly-defined process

attributes. The values to the left of the heavy line are the

process attributes determined in the Harris mass spectrometer

and are repeated from Table IA.

TABLE IB

Harris Mass Spec Commercial Lab

Repeated from Table IA

Process Process

Mean UCL Mean UCL

Eutectic 1527 2623 504 1602

Epoxy 2267 4541 774 2967

80
It is essential to remember that samples analyzed at the

commercial lab are from the same uninterrupted manufacturing

process, operating essentially in control. The only apparent

assignable cause for the shift in means and UCLs is the change

in analytical equipment.

During the time that the SPC data-gather ing plan was

being fulfilled by data from the commercial laboratory, the

mass spectrometer in the Harris laboratory underwent extensive

refurbishment

.

This included:

• Installation of a computer for instrument control and

data acquisition.

Change-out of all valve tips, seats, and gaskets.

Re-build of all pumps.

Re-build of controller boards.

Re-build of all seals and flanges.

Cleaning of all seals and flanges.

Installation of new stainless lines and complete

assurance of leak-free instrument operation.

All of the hardware upgrading resulted in a like-new

analytical system. It returned to service more than 2 months

after breakdown. With the mass spectrometer up, the SPC

analysis plan reverted back to the Harris equipment before

full consideration could be given to the import of the

differences in analytical equipment evident in Table IB.

81
VI . Another Statistical Definition of the Process

As data accumulated from the ongoing operation of the

re-built Harris instrument, yet a third picture of the

moisture attribute emerged.

A

.

Eutectic Process

Fig 8 is a histogram of the 56 samples of the eute

process measured from the return to service of the Harr

machine to the writing of this text. While the re are

insufficient samples to judge whether the data is normally

distributed, the histogram gives a good picture of how the

re-built mass spectrometer sees the moisture distribution in

the process. It is especially interesting to overlay Fig 8

with Fig 5, to compare the distributions defined by the

commercial laboratory instrument and the re-built Harris

instrument

.

Figs 9 A and 9B are the xR charts of the process as

measured by the re-built Harris instrument. The process' mean

moisture content is now defined as 865ppmv, with UCL of

3087ppmv.

B . Epoxy Process

Figs 10A and 10B are the xR charts for the epoxy

process. The 44 data points constitute a statistically valid

chart, indicating a new mean for the epoxy process of 1478ppmv

with a UCL of 4067ppmv.

C . The "Re-revised" State-of-the Process

Table IC summarizes the newly-redefined process

attributes. Values to the left of the heavy line are repeated

82
from Tables IA and IB for ease of comparison.

TABLE IC

Harris Mass Spec

Repeated from Table IA

Commercial Lab

Repeated from IB

Harris Re-built

Mass Spec

Process Process Process

Mean UCL Mean UCL

| Mean UCL

Eutectic 1527 2623 504 1602 1

864 3087

Epoxy 2267 4541 774 2967 1 1478 4067

VII. Comparison of Means for the Analytical Equipment

The "inter-ocular exam" of Table IC and Fig 11 certainly

suggests that the moisture attribute of this sealing process

depends upon which analytical system is being used to characterize it.

To compare whether or not these various means are equivalent statistical tests of the data were carried out. A

. 5 t-statistic was computed for each pair of means to be

compared by _

Where Y = mean for a given set of data

s = the variance for that set of data

n = the number of points in the data set.

This formula comprehends large differences in s and n

between data sets, and also is not especially sensitive to

whether or not the data are normally distributed.

Summary statistics for the data sets are in Table II.

83
Table II

Eutectic Historical

Summary Sta

Harris Values

t ist ics

Data

Set

I n362

Mean

1527

Variance

275975

Process Commer ical Lab 1

1

38 504 442971

Re-built Ha rris Instument 1 1

1

56 865 949221

Epoxy His tor ica

1

Harris Values IV 173 2267 1022717

Process Commercial Lab V 16 774 486478

Re-built Ha rris Instrument VI 44 1478 1643908

Table III records the results of the comparison of means.

Table III

Equality of Means

Comparing Computed Critical Value Means

Data Set t-Sta t is tic o f t Equivalent?

Eutectic I with II 9 . 18 1.96 no

Process II with III 2 . 13 1.96 no

I with III 4 . 97 1.96 no

Epoxy IV with V 7 .83 2 . 12 no

Process V with VI 2 . 70 2 . 12 no

IV with VI 3 .79 1.96 no

The critical va lues of t are for a = .05 and are for

degrees of freedom d ic tated by the least value of n for the

two data sets being compared. The value of 1.96 is common for

all degrees of freedom >29. In all cases, the computed

t-statistic exceeds the critical value of t. In all these

cases, it can be stated with 95% confidence that no two

compared data sets are the same. The only apparent assignable

cause for this is the change in analytical equipment.

It can be seen from Fig. 11 that not only do the process

means shift with changes in equipment but so do the UCL ' s

.

The shifting UCL ' s indicate a change in the variability of the

moisture attribute. And it is interesting to note that when

the re-built instrument resumed operation, the process mean

84
for eutectic was substantially lower than originally, but its

associated UCL was higher.

The total variability of any attribute is the sum of its

natural variability plus the variability inherent in the

equipment used to measure the attribute. In the data summarized in Fig 11 there is apparently an increase in the variability of the sealing process or of the measurement or of both.

Given the lack Qf "standard" packages, we do not have a good definition of measurement variability. The minimum

amount of measurement variability is defined by the reproducibility of the moisture values read on bursts of humidified

N

2

whose moisture content has been determined by a calibrated dewpoint hygrometer. For O.lOcc bursts of N

2

humidified

to 5000 ppmv moisture, the variances for replicate bursts

observed in the Harris instrument were 18068 and 19233 before

and after re-build, respectively. These variances are much

smaller than all those for the various process measurements in

Table II (all are >275,000).

From this it is assumed that much of the observed

variability is process variability. It must also be assumed

that measurements after the re-building of the instrument

indicate a lower process mean, but that the process variability for that particular measurement window is greater. This

is observable for both the eutectic and epoxy process.

VIII. Summary and Conclusions

It is clear that analytical mass spectrometers applied to

the nontrival problem of measuring water vapor in small

85
enclosures must be absolutely optimized with respect to hardware. Optimized vacuum practices and equipment maintenance,

with assurance of no virtual vacuum leaks, account for the

apparent reduction in process mean obtained by re-building the

equipment

.

But even after instrument improvements, the eutectic

process UCL is 3087ppmv, higher than that attained in the old

system. This could be a consequence of a higher natural

tolerance for the segment of the process represented by the 56

samples, even though the mean is lower.

Even though the re-built instrument is now operating to

the very best of its mechanical ability, a discrepancy remains

between it and the commercial laboratory instrument. This

discrepancy:

(1) is statistically significant.

(2) is specif ica t ion-conf ormance significant. That is, if

the process is qualified at the commercial laboratory it

would produce parts capable of meeting an engineering

maximum tolerance reduced from 5000ppmv to 2000ppmv, but

the same process qualified by the manuf acturer ' s own

analytical equipment would not conform to an engineering

maximum tolerance* reduced from 5000ppmv to only 3000ppmv.

There is a need, in future efforts to build "standard"

packages for mass spectromet r ic calibrations , to consider both

the accuracy and precision of water vapor determinations.

This is because the notion of process control, which is so

86
critical for both cost-effective as well as inherentlyreliable processing, must comprehend knowing both the value of

the process mean (accuracy) as well as the variability

(precision) with which measurements are made, so that true

process variability can be known. "Standards' must be

prepared with this in mind, and participating laboratories

must be willing to measure enough replicate samples to

establish precision. It is not sufficient to assume that

calibration bursts identify all measurement variability,

because that procedure does not duplicate the physics of

moisture release from the component materials of sealed

packages into the mass spectrometer.

Acknowledgments: The invaluable

Dr. Jack Linn with mass spectome

acknowledged along with the work

the manuscript.

efforts of Steve Slasor and

ter operation are gratefully

of Caroline Connor in typing

87
References

1. White, M.L., Striny, K.M. , and Sammons, R.E., "Attaining Low

Moisture Levels in Hermetic Packages," in Proceeedings of the

20th Annual Reliability Physics Symposium, March 30-April 1,

1982, San Diego, CA, pp. 253-259.

2. Proceedings of the RADC/NBS Workshop, Moisture Measurement and

Control for Semiconductor Devices, III, May, 1984, Sessions

IV-VII. pp. 180-313.

3. Loo, M.C., "Influence of Seal Glass on the Hermeticity of

Cerdip Packages," Solid State Technology, 29(8), August, 1986,

pp. 163-165.

4. Elo, R.B., and Massoletti, A.M., "Control Chart Analysis of

Moisture in CERDIPS," in Proceedings of the RADC/NBS Workshop,

Moisture Measurement and Control for Semiconductor Devices,

III, May, 1984, pp. 24-39.

5. Parr, W.C., "Economical and Informative Experimentation for

Process Improvement and Development," Harris Semiconductor

technical memorandum, 1985, pp. 1.77.

88
PftN PDISTIFE

Figure 1. Histogram, Eutectic

Seal Process, 1985

— Values

- UCl — a

- LCL

Figure 2A.

x Chart, Eutectic

Seal Process, 1985

RANGES

uCL

CLLCL

Figure 2B.

R Chart, Eutectic

Seal Process, 1985
pm/ fUISTURF

Figure 3.

Histogram, Epoxy Seal

Process, 1985

RANGES

UCL

LCL

Figure 4B.

R Chart, Epoxv Seal

Process, 1985
NLfBER

OF

SPECIhtNS

60

50-

AOpprv rnisnFE

Figure 5.

Histogram, Eutectic Seal

Process, measured by

commercial laboratory.

—o— Valutt

UCL Figure 6A — a

La

x Chart, Eutectic

seal Process,

measured by

commercial laboratory.

3000-r

i —o— RANGES

UCL

Figure 6B

R Chart, Eutectic

Seal Process,

measured by

commercial laboratory.

91
Figure 7A.

x Chart, Epoxy Seal Process, measured by

commercial laboratory

„

—o— RANGES

IJCL

LCL

Figure 7B.

R Chart, Epoxy Seal Process, Measured by

commercial laboratory.

92
PPft/ MOISTURE

Figure 8.

Histogram, Eutectic Seal

Process, measured in

original mass spectrometer after major instrument renovation.

Valuts ua

a

La

Figure 9A.

x Chart, Eutectic

Seal Process, measured

in original mass spectrometer after fl&jor

instrument renovation.

4000-r

2733

Figure 9B.

GANGES

LiCL CL

R Chart , Eutectic

Seal Process, measured

in original mass spectrometer after major

instrument renovation.

93
Figure 1UA. —>— Values

UCl

CLLCL

x Chart , Epoxy Seal

Process, measured in

original mass spectrometer after major

instrument renovation

Figure 10B.

R Chart, Epoxy Seal

RANCES Process, measured in

UCL original mass speech- trometer after major

LCL instrument renovation.

Tabla U Tab la IB Tabla IC

Figure 11A.

Chronological Plot of

Means and UCL's for

Eutectic Process.

94
MOISTURE

PP

tol

UCL

HAULIS com LAB HAUIS

JQP 5

Ftb - April '86 r#-builc inae Tibia LA Tabla IB Table IC

Figure 1 IB Chronological Plot of Means and

UCL's for Epoxy Process.

95
2.2 Laboratory Correlation and Survey Experiment:

A Status Report

Benjamin A. Moore

Rome Air Development Center

RBRE

Griff iss AFB NY 13441-5700

Didier Kane

Clarkson University

Potsdam NY 13676

Abstract: The presence of moisture within a microelectronic package can

have a detrimental effect upon the reliability of the enclosed device. In order to screen these reliability hazards from military systems, a test

method, 1018 (Internal Water Vapor Content) was developed and made part

of MIL-STD-883, Test Methods and Procedures for Microelectronics. The

most prevalent method of measuring moisture is mass spectrometric

quantitative analysis (Procedure 1 of Method 1018). However, in order to assure accurate and reproducible mass spectrometric moisture

measurements from commercial analytical facilities, several laboratory

correlation experiments have been performed. This paper briefly details

past correlation results and then presents the status of the current

analytical laboratory comparison. Preliminary results indicate correlation

for most samples, at approximately 2500 ppm among all laboratories

reporting. However, unresolved problems and incomplete data prevent

final conclusions from being made.

Key Words: Moisture; mass spectrometry; correlation; Method 1018;

calibration; standards.

1. INTRODUCTION

The purpose of this work was to determine (1) whether analytical correlation at 5000

ppm moisture has been maintained by both commercial and in-house (semiconductor

manufacturers) analytical facilities and (2) if correlation could be established in the

2000-3000 ppm range. This would justify proposed changes to the upper acceptable

limit for moisture in Method 1018 and Methods 5005 (Qualification and Quality

Conformance Procedures) and ,5p0& (Test Procedures for Hybrid and Multichip

Microcircuits) of MIL-STD-883.

The Product Evaluation Branch (RBRE) of the Rome Air Development Center (RADC)

is the responsible Government agency for the technical aspects of Method 1018 and, as such, deals with such issues as the determination of laboratory correlation to assure accurate and precise analyses and the implementation of changes to Method 1018 to

reflect current military requirements. It is also responsible for the development of

nondestructive, low cost alternative moisture test methods, resolution of conflict

analyses from Government certified laboratories on military qualification samples,

the investigation of gas matrix effects (i.e., the presence of hydrogen and/or oxygen)

96
on analytical accuracy, the certification of the technical competence of analytical

facilities to accurately and precisely measure moisture, the expansion of

certification volume range to include VHSIC, VLSI, and hybrid devices, and the

evaluation of the effects of organic material on moisture analysis techniques.

Field experience and failure information derived from the Government-Industry Data

Exchange Program (GIDEP) Alert system suggests several changes should be made to

Method 1018. These include (1) assurance of no cold spots (moisture condensing

surfaces) in analytical systems by requiring all gas transfer passages be baked at a minimum of 100 C, (2) pre-analysis bake of ail parts for 12 hours prior to hot insertion

into analysis system (not required for batch systems with inherent 12 hour preanalysis bake) to assure consistent device preconditioning and (3) lowering the upper

acceptable moisture limit to 2000-3000% ppm . Moisture aided failures have been

observed at approximately 2300 ppm . In order to improve the test method itself,

several changes will be requested. These include formalizing the concept of

correlation samples (in lieu of the now required bienniel examination by the

Government dewpoint hygrometer), reporting of all gases detected within the

accuracy limits of each analytical laboratory, and inclusion of a requirement for

documented procedures to detect and, if necessary, compensate for gas matrix

effects. These proposed changes were important factors in the design of the current

correlation experiment.

2. PROCEDURE

2.1 Prior Experience

Two prior correlation experiments have been conducted. The first, conducted in late

1979, served as the basis for implementation of Method 1018. As shown in Table I, RADC and the commercial analytical facilities demonstrated excellent agreement

with standard deviations of less than 10% of the mean values. In order to assure continuing analytical correlation, to answer questions raised by unofficial and ill- conceived laboratory comparisons, to investigate correlation at moisture

concentrations other than 5000ppm , and to evaluate different packaging

technologies Lor correlation sample production, a second experiment was conducted

in late 1981. Table II summarizes the results of this recertification program.

Analyses of Lot RA, which were samples remaining from the original 1979 correlation

experiment, resulted in an overall mean differing only 82 ppm from the originally

determined value. Lots RC (X=9358), RE (X=6483), and HA (X=1^63) correlated close

to their target values. Lot HA samples were ceramic dual-in-line packages

(CERDIPs) supplied by Harris Corporation, several of which contained an

interdigitated sensor that recorded a moisture mean value of 1562 ppm . Lot RD

(X=2628) samples contained approximately 2Vi times the target moisture value but

analytical results generally exhibited good correlation.. Results of this experiment

demonstrated both the feasibility of producing a suitable moisture standard in the

2000-3000 ppm range and the possibility of extending laboratory correlation from a

single point to a range of values. Subsequent observations made on remaining Lot HA

samples have shown a gradual upward drift in moisture % content (which can be

thermally accelerated) to approximately 2500-3000 ppm^.

97
2.2 Experimental Plan

In order to (1) satisfy the biennial reexamination requirement dictated by Method

1018; (2) determine the degree of retention of laboratory correlation after the five

year period since the last formal comparison; (3) provide data to support proposed

changes to Method 1018; and (4) determine the state-of-the-art in mass spectrometric

moisture measurement, a comprehensive experiment was designed and initiated in

early 1986. All participating laboratories were required to be calibrated over the

range of 1000-10000 ppm moisture for the sample volume range to which each

laboratory had been certified. All testing, unless otherwise specified, was to be done

in accordance with Method 1018.2 of MIL-STD-883C. Five samples were included in

each sample group with a requirement that, if after testing three samples the data

spread was greater than 20%, the remaining two samples would also be analyzed.

The experiment was divided into three phases. Each laboratory was to complete the

initial phase, report results, and wait for instructions to proceed to subsequent phases

of the experiment. The primary experiment consisted of two groups of TO-3 (B, Bl)

metal cans, two groups of TO-18 (A, Al) metal cans, and one group of CERDIP (H)

samples. Groups A and Bl were designed and packaged to contain approximately 3000

ppm , while groups Al and B were targeted for 2300 ppm . These devices were

procured by the Government from the Raytheon Corporation. Group H was the

remaining samples of Lot HA of the 1981 correlation experiment, and, as previously

mentioned, contained 2300-3000 ppm moisture. This mix of samples featured three

different cavity volumes and two different sealing technologies. Phase II devices

included a variety of TO type metal cans of different volumes and moisture

concentrations. These devices were supplied by National Semiconductor. A Latin

square type experiment was designed with each laboratory to do some elements in the

test matrix. A similar test plan, with additional devices from National, was used in the Phase III experiment. The Phase III portion of the correlation experiment,

required of commercial laboratories only, was designed to evaluate the effect on

analytical correlation of a preanalysis bake-out at various temperatures for various

lengths of time. As seen in Table III, Phase I of the experiment is not yet complete.

Therefore, technical data that is presented in this paper is preliminary and subject to

change upon inclusion of data from laboratories whose testing is incomplete and any

additional testing that may be required. The purpose of this paper is only to present

a status report on the laboratory correlation and survey experiment. Official data

will be presented to the appropriate Joint Electron Device Engineering Council

(JEDEC) subcommittee(s).

Note that the Intel Corporation voluntarily requested removal from Method 1018

suitability shortly after samples were sent to each laboratory.

2.3 Results

An overall summary of available Phase I results is presented in Table V. Laboratory

letter designations for this experiment are consistent within this experiment but

differ from those used for the earlier correlation experiments (79 and 81). The results

from Lots B, Bl, and H are very encouraging and will be discussed in greater detail

later in this paper. Results from Lots A and Al (the smallest volume samples in this

phase of the experiment) are puzzing. Within each laboratory, analytical results are

consistent but there is no correlation among laboratories. A preliminary

investigation of this problem centered on comparison of the analytical techniques

98
used by each laboratory. This comparison of measurement techniques is seen in Table

V. The results from those laboratories with non-instantaneous moisture

determinations are closer to the correlation sample target values. Data from Phase

II of the correlation experiment should help resolve this problem. Additional low

volume sample testing is being planned.

The results from Lot B are presented in Table VI. A graphical representation of this

data is seen in Figure 1 with a comparison of the overall laboratory mean _+ 20%.

Each individual laboratory mean moisture value is seen in Figure 2. All laboratories

correlate within +20% of the overall mean of 2562 ppm, . Results for Lot B1 are

similarly detailed in Table VI and Figures 3 and 4 with correlation within _+ 20% of the

overall mean of 5462 ppm (lab E is actually -20.5% below the mean value). Lot H

results are presented in “fable VIII and Figures 5 and 6. Again, correlation within

+20% of the overall mean of 2517 ppm is demonstrated. A comparison of the range

of all individual determinations within +20% of the overall mean is made in Table IX.

Data is presented as follows: minimum value, maximum value, number of samples

outside +20% range, + symbol representing direction of deviation. Each

< represents values less than 20%, while each> is for values greater than 20%.

3. OBSERVATIONS

Several disconcerting problems, in addition to the non-correlation of Lots A and Al, arose during the correlation experiment. One laboratory did not read the instructions

before proceeding with Phase I. Since the laboratory did not calibrate for moisture

from 1000-10000 ppm as required, all analyses had to be repeated. Two laboratories

tested on systems that were operating improperly. In one case, the data acquisition

system had a known intermittent fault. The other system was not allowed to attain

equilibrium conditions after an extended repair cycle. In both cases all analyses had

to be repeated. Some laboratories questioned the importance of these samples and

delayed analyzing them. System downtime with long repair cycles hampered several

laboratories. Lack of in-house technical and maintenance support, along with less

than timely responses from analytical equipment manufacturers, severely hindered

non-commercial laboratories, including RADC, from completing the experiments.

The Government has initiated several actions to reduce the severity of these

problems. Correlation samples are being used to audit analytical laboratory

performance and are required for maintenance of laboratory suitability to perform

moisture analysis on parts for qualification for military applications. The Defense

Electronic Suppy Center (DESC), the Government agency that grants laboratory

suitability to perform Method 1018 analyses, will suspend the suitability of any

laboratory not performing correlation analyses in a timely manner. In order to solve

correlation problems, small volume samples from alternate sources are under

consideration and investigations into alternate calibration equipment and procedures

are proceeding.

Several other approaches to determining and maintaining laboratory correlation are being considered. These include (1) bli-nd surveys through a contractor with random

sampling of all laboratories, (2) direct purchase of analyses with subsequent "naming

of names" when reporting data, and (3) direct or indirect supply of samples to be

included with each analysis lot. The last approach could serve as the basis for inlaboratory statistical quality control measures and could possibly be accomplished by

99
encouraging the development of independent source of samples (indirect supply).

Communications between the Government and all analytical facilities need to be

improved. More frequent visits, primarily technical in nature, to each laboratory are being contemplated. Plans for more complex audits of each facility are now being

discussed by RADC and DESC.

4. CONCLUSIONS

Since Phase I of the correlation experiment is incomplete, only tentative conclusions can be made based upon available data. Maintenance of correlation at 5000 ppm

moisture has been demonstrated by analytical results of Lot Bl. In addition,

correlation of moisture measurements at 2500 ppm has been established by

analytical results of Lots B and H. However, uncertainty remains as to the volume

range of correlation among laboratories due to the poor results of analyses o-f Lots A

and Al. Additional analyses of low volume ( ^

0.01-0. 02cc) samples will be required

to resolve this problem. Finally, more Government-laboratory interaction and

dialogue is required to assure continued correctable moisture measurements of

microelectronic package ambients.

REFERENCES

1. B. A. Moore, "Correlation of Analytical Facilities," 21st Annual Proceedings of

the International Reliability Physcis Symposium, Phoenix AZ, April 1983.

2. GIDEP Alert HA-A-83-01.

3. B. A. Moore, "Method 1018.2 Certification Results," Moisture Measurement and

Control Workshop, National Bureau of Standards, Gaithersburg MD, November 1980.

4. B. A. Moore, "Method 1018 Moisture Measurement: The Evaluation of Analytical

Correlation," Journal of Environmental Sciences, January/Febrauery 1987.

100
TABLE 1 TABLE IV

fflHDO 1018 ffiism ANALYSIS CfBTlFJCATIDN

10-18 Slillffl MRESULTS usm collation me mwbesults- pis? i MOISTURE IN PPHv

LABORATORY TRIALS DATES MEAN ddiiiv STD DEV. ddidv

RADC 13 4 5280 436

C e 2 5160 403

D 7 1 4767 156

B 5 1 5398 443

A 4 1 5335 282

LAE A Ai E El H

B 4456 32B? 234? 5875 2923

C 6200 5300 3025 5633 2067

D 10149 5519 2529 5331 2653

E 2867 2240 2336 4345 2272

G 9000 6500 2567 5625 2733

KEAN .... — 2562 5462 2517

X-POJ — 2050 4370 2014

X4?0I — — 3074 6554 3020

MEAN' VALUE, ppov: 5lBfl

mu ii

MT1H0D 1018.? mmmPROGRAM results

MOISTURE JN PPMV

LAB RA BE BL BE BL HA

RADC 5327 4535 10060 2663 6040 1570

C 5G70 5730 9900 2800 7800 1920 D

A 5631 4453 3317 em Si ii

2062

1901

MEAN 5270 5568 9358 2626 6483 1863

X-POX 4216 4454 7486 2102 5187 1491 X4?0I 6324 6602 11229 3153 7780 2236

TARGET

VALUE 5200 3000 10000 1000 6000 1562

table v

COMPARISON Or nmm 1ECHB01B LOIS A £ 11 MOISTURE IN PPmy

LAE A MEAN Al MEAN SAM^LF BAKE INSTANTANEOUS

B 4458 3282 SINGLE N N: OUASHOUILIBRIUM

C 6200 5300 BATCH Y Y: RATE Of RISE

D 10149 5519 SINGLE N Y: MAXIMUM

E 2867i 2240 SINGLE N N. LIMITED INTEGRATION

8 9000 6500 BATCH Y Y: MAXIMUM

« NO VALUES P5000. NO VALUES §500

TABU III

flHOD 1018 CORRELATION EXPERIMENT STATUS

TABU IX

MOISTURE IN PPMV

LAE EHASLI PHASE II PHASE ITT

RADC IN PROCESS NOT STARTED MOT STARTED

ATT COMPUTE MOT STARTED

6AS complete NOT STARTED itoT STARTED HARRIS 'COMPLETE NOT STARTED

INTEL « i i ITAS COMPLETE MOT STARTED MOT STARTED

NORTHROP IN PROCESS MOT STARTED

ORS OMPlEIE NO! STARTED NOT S1AHTEU

PC IN PROCESS NOT STARTED MOT STARTED

* DROPPED METHOD 1016 SUITABILITY

LAE. B 12050-3074) Bl 14370-6554) H (2014-3020)

B

CO

E8

1990-2750- 1< ?600-3400-2>

2462-2593-0. 1760-2930-K

2500-2700-0

5420-65B0-l> 5300-5800-0 51B9-5430-0

3970-4950-2< 5700-5900-0

2770-3040- 1> 1B00-2200-K

257-4-2764-0 1B60-2600-K

2500-3000-0

101
1«Li VI lAiJ V||

i imm1 CffEELATIOK AND SI® [KHi

KuISlJftE IN PPM !OI aiS: L0

] 8 PHAS: 1 Ulffiffl' OKLATIOK AS SR^r fffHIOi RGHT TS' HCISltK IK Phi;

LAS MEAN sim LAB MEAN Sim

B 2342 334

B 5675 496

C •3025 330

C 5633 289

D 2529 66

D 5331 126

E 233B 470

F 4345 441

6 2567 115

G 5625 96

X 2562 392

X 5462 * 723

LOT B AVERAGE MOISTURE DETERMINATIONS

W& KAN* 2562

f I6URT J E2 KAN* 546?

LOT Bl AVERAGE MOISTURE DETERMINATIONS

FI6URI 3

LOT B ACCEPTABLE MOISTURE LIMITS (+/- POX) LOT B) ACCEPTABLE MOISTURE LIMITS (+/- POX)

CD LABORATORY KAN JM TOTAL KAN /* 201

FI WIRE 2 CD LABORATORY KAN ZM TOTAL KAN /- 201

FI6URE « 102
TABLE VIII

PHASE I LABORAilRY CORRELAIION A.ND SURVEY EXPHIHEHT RESULIS: LOT H MOISTURE IN PPHV

LAft MEAN SIQQEV

B 2933 139

C 2067 231

D 2663 109

E 2272 320

G 2733 252

X 2517 378

•Can* 2517

LOT H AVERAGE MOISTURE DETERMINATIONS

FIGURE S

LOT H ACCEPTABLE MOISTURE LIMITS (+/- 20X)

O LABORATORY *an TOTAL NEAN /- 201

FIGURE 6103
2.3 ACCURACY AND REPRODUCIBILITY OF THE MASS SPECTROMETER/CAROUSEL

METHOD FOR MOISTURE ANALYSIS

Leslie J. Rigby,

STC Technology Limited

London Road, Harlow, Essex, CM17 9NA, UK

A computer controlled mass spectrometer system with a carousel sample

chamber has been carefully evaluated using prefilled T05 packages

containing 1850 and 4300 ppmV of water vapour in nitrogen. The filling

gases and the mass spectrometer calibration gases were all monitored with

a cooled mirror hygrometer with calibration traceable to the NBS standard.

Over 36 samples of each type were analysed with excellent consistency

over a period of 6 months. The ion integration method was used with a cut off point after 3 minutes. Samples and standards were alternatively

measured and for both the typical standard deviations were 4 and 15% at

the high and low moisture levels respectively.

The mass spectrometer sensitivity to water was found to increase with

both moisture and oxygen content in the sample. These and other factors

could contribute to measurement errors. Standard samples which contain

similar contents of both water and oxygen in the sample are required for

accurate measurement. Accuracy over a range of moisture levels requires

extensive standardisation procedures, but invariance in both the internal

hardware used and the sampling and measurement procedures yielded

calibration data which were essentially consistent over the six month

period

.

These and other measurements have revealed the advantages and

disadvantages of the carousel procedure. A modification to the carousel

method will be described in which the benefits are retained but the

disadvantages are reduced and in, some cases eliminated.

104
1 . INTRODUCTION

The EEC Community Bureau of Reference (BCR) has organised an interlaboratory comparison exercise within the EEC for moisture

measurement in encapsulated packages. This report refers to the STC

contribution to the exercise which was to prefill packages at two

moisture levels, to test the filling procedure by mass spectrometric

analysis of specimen packages, and to participate in the exercise by

measurement of a selected sample of the final batches of packages. 120

packages were required at each moisture level which was set between 1000

and 2000 ppm at the low moisture level and between 4000 at 6000 ppm at

the high moisture level. Close control was required to ensure that each

batch had a minimum spread in moisture content. The final batches were

produced in one run after' an overnight equilibration of the moisture

level.

A recording mirror dew point hygrometer (Michell 4000)

1 was provided

by BCR to monitor the filling gas humidity and to check the calibration

of the mass spectrometer during the analysis of the final batches.

2 . ENCAPSULATION

A schematic diagram of the encapsulation equipment is shown in

Fig. 1. This is the equipment normally used to dry fill high reliability

packages but it has been modified to allow a controlled degree of

moisture to be introduced into the filling gas. Dry nitrogen (moisture

content 10 ppm) was split into two streams which were controlled by

rotameter flow gauges. The moisture content in the wet stream was

. . . » o derived from deionised water m quartz with +0.2 C temperature

control. A high current weld sealed each nickel can to the corresponding

gold plated kovar base. A few devices were rejected by visual inspection

of the welded rim after assembly. The assembly enclosure requires a throughput of 12 litres of gas per minute and a sampling tube was introduced close to the encapsulation head to draw off 0.5 litres/min. to

satisfy the requirements of the Michell hygrometer. Tables 1-5 list

the measured water contents and will be discussed in more detail in

Section 5.

105
For the low level samples (1000-2000 ppm) an initial batch of 12 packages

were sealed in nitrogen with a dew point of between -16.80 and

o -16.90 C. This corresponds to a moisture level of about 1400 ppm but

the mass spectrometer analysis produced an average moisture content of

less than 600 ppm (Table 1). The discrepancy was traced to the cooling

rate setting on the hygrometer. The value recommended in the instruction

manual was found to be too low and subsequent measurements were made to

verify that the dew point reading did not increase as the cooling rate was increased. In the repeat experiment, a dew point of between -15.46

and -18.02 resulted in an average mass spectrometer reading of 1400 ppm

(Table 2). The final batch of 120 devices had a dew point range of from

-16.14 to -16.58 (1750-1690 ppm) and a measured content of

1850 ppm (Table 3).

The high level condition (4000-6000 ppm) required only 1 trial run

. o (Table 4). A dew point of between 1.77 and 2.06 C gave an average

measured content of 10,000 ppm in the first six packages. Lower dew

points of from -0.98 to -3.84 produced a measured package content of

. . o 8000 ppm. The humidity was reduced to an overnight dew point of -5 C

and for the final batch of 120 packages, a dew point range of -4.5 to

-5.1°C (4400-4300 ppm) resulted in an average content of 4300 ppm for

the measured packages (Table 5).

3. PROCESSING AND DISTRIBUTION

All of the 240 packages from the final encapsulation were soaked in

3 atmospheres pressure of helium at ambient temperature for 72 hours

prior to distribution. 12 equally spaced samples were then removed from

each group of 10 within a batch of 120 for immediate analysis with the

STL mass spectrometer system. A second set of samples were similarly

taken for a repeat analysis 4 weeks later. The remaining samples were randomly chosen, one per decade, so that the other participating

laboratories would receive an unordered distribution from the samples

available

.

106
4 . GAS ANALYSIS

. 2 . The procedure used has been described previously and remained

consistent throughout. Twelve cans were loaded the previous afternoon

O • and the whole system baked at 100 C in a background pressure reducing

to between 20 to 30 nanotorr by a turbomolecular pump. The mass

spectrometer background was less than 5 nanotorr.

3 A Protimeter recording mirror dew point hygrometer was used to

monitor the water content in the standard gas. A computer algorithm was used to read the output from this hygrometer and calculate the % water

content. Comparison with the certified Michell Hygrometer revealed that

o the Protimeter dewpoints were within 0.1 C for mirror cooling of up

o to 15 C below the ambient temperature.

Prior to each analysis, 0.1 ml of room air was admitted to water saturate

the surfaces of the sample chamber which was then evacuated for a further

5 minutes. Room air was also used to determine the mass spectrometer

sensitivity to water vapour. A 0.5 1/min flow was generated with a small

compression pump. This air stream was warmed by passing through the

o 100 C heated chamber m a 7 mm I.D. stainless steel pipe 60 cm long

and onto the two calibration volumes of 0.1 and 0.01 ml. A short exit

line passed initially through the Michell hygrometer and then to a Protimeter recording mirror hygrometer which was routinely used to

calibrate the mass spectrometer system.

For all the data reported here, samples and standards were expanded into

a chamber volume of 140 ml and transferred via a vernier needle valve

with the same preset conductance to the mass spectrometer. 19 cyclic

readings of all the relevant ion currents were completed in 3.3 minutes.

During this time, over 99% of each original gas sample had passed through

the mass spectrometer ion source. The room air standard gas was initially analysed until a consistent sensitivity to water was obtained

and then remeasured after each sample to check for any variation in

instrument sensitivity.

107
Room air was used to measure water vapour sensitivity as it was readily

available and the water content was consistent showing only a slow

increase over a 5 hour period. More importantly, typical dew points were

o above 5 C and allowed the Protimeter unit to be used without

modification (Lower dew points would require additional cooling of the

mirror and the negative dew point readings would not be understood by the

digital microprocessor). However, the range of moisture levels can be

covered by using air volumes of 0.1 and 0.01 atm. ml. (Section 5, para

1) . This is valid because under the conditions of molecular flow used to

transfer the sample gas to the mass spectrometer ion source, the partial

pressure and the ionisation cross section of water should be unaffected

by the predominant gas pressure. This statement was subsequently

partially verified (Section 5, para 6).

5 . RESULTS

Table 8 shows specimen results of the computer acquired data for

standards and samples. The 0.1 ml air standard was used with the high

level samples and the 0.01 ml standard with the low level samples. The

peak ion current and the area indicate that the quantity of water in the

0.1 ml standard (Table 8) is approximately twice that in the high level

sample (Table 11) whereas the reverse is the case for the 0.01 ml

standard and the low level sample . The decay curves for these smaller

quantities of water are much shallower and indicate the greater

difficulty in measuring smaller water concentrations. Under these

circumstances, area measurement is preferred to maximum peak height for

water content measurement.

Standards and samples were alternately measured during each run. Tables

1 to 7 give the essential data for water content. The water content in

all the standards has been calculated from the Protimeter hygrometer.

108
The average values from Tables 1 to 7 (corresponding to runs 1 to 7) may

be summarised as follows:

Sample

Sensitivity % deviation Water % devia

from Air content

Standard

1 . Low Level Test 1 0.25 6.7 0.053 16

2. Low Level Test 2 0.29 6.8 0.18 11

3. Low Level 1 0.25 6.3 0.19 14

6. Low Level 2 0.21 7.7 0.18 13

4. High Level Test 1 0.38 2.9 1.0 8.5

0.80 3.0

5. High Level 1 0.36 4.9 0.43 3.5

7. High Level 2 0.32 3.0 0.41 5.7

The sensitivity values are all relative to nitrogen (mass 28) and are all

significantly less than 1. This may in part result from a lower

ionisation cross section for the water molecule but mainly follows from

water loss by surface adsorption. Table 8 indicates the much slower

transfer of water to the mass spectrometer during the measurement time

and a lower sensitivity for water is therefore not unexpected. Both

measurement and filling consistency are good at the low level (1800 ppm)

and significantly better at the higher level (4200 ppm).

Water calibration with moist nitrogen was carried out at a later

date over a 5 day period (7-11/1/85) with a General Purpose Humidifier.

Dry nitrogen is saturated with water at an elevated pressure (p^) . The

water content at ambient pressure (p

Q

) may then be calculated from the

saturation vapour pressure (p^) and the pressure drop:

%H_0 = 100 p .p /p, .p = 100 p /p.

2 s o

r l o

r

s r

i

The actual moisture levels obtained were measured with the Michell

hygrometer which was used to calibrate the mass spectrometer. As with

the previous work, room air was used to presaturate the walls of the

vacuum system prior to sample admittance. The relative sensitivities to

water from air and nitrogen standards were as follows for % water in

volumes of 0.1 atm. ml.:

109
%H^0 In air In nitrogen

0.1 0.21, 0.21, 0.25, 0.29 0.15 0.11

0.2 0.13 0.30 0.14 0.19

0.4 0.20 0.21

0.6 0.20 0.28 0.23

0.8 0.38, 0.36, 0.32 0.28 0.25

1.0 0.27

The air calibrations were all on different days as indicated in the text

and were nominally at two fixed points. The 0.1% level in air was in

fact 1% in 0.01 atm. ml.

The above comparison indicates that a reduction in sensitivity of about

50% occurs when nitrogen is used instead of air. Oxygen is a very

4 reactive gas at metal surfaces and active gases have been shown to

replace more labile molecules^ Chemisorption of oxygen at surface

sites could result in the release of preadsorbed water molecules and

assist water transfer to the mass spectrometer. The deliberate act of

conditioning the walls of the sample chamber with moist air would

inevitably promote this effect.

Sample consistency in gas content, argon and carbon dioxide for Runs 4-7

were as follows with the % deviation in brackets

4-High level 5-Low level 6-High level 7-Low level

Content/m. bar .

1

0.218 ( 2.1) 0.196 ( 1.3) 0.213 ( 1.4) 0.216 (2.8)

% Argon 0.017 ( 2.8) 0.016 ( 7.9) 0.017 ( 6.4) 0.014 (3.6)

% CO 0.029 (15) 0.035 (13) 0.032 (37) 0.033 (8.8)

2

These data give a good indication of the precision of measurement for the

permanent gases. The higher CO variability may result from

• 2 variations in local heating during can welding.

All the calculations referred to above are based on an area integration

of all the ion current/time data points generated by 19 repetitive

cycles. The sensitivity of the mass spectrometer to water, hydrogen,

helium, oxygen, argon and carbon monoxide relative to nitrogen, which was determined from area measurements, has also been used to calculate sample

110
concentrations from the ion current peak heights. In all cases, these

are the initial ion current values and for argon and carbon dioxide the

peak height calculation is in good agreement with the peak area determination. This indicates that the mass spectrometer sensitivity is

the same for both calculations. For water, peak height calculations

based on the peak area sensitivity result in a significantly lower %

water. This is not surprising since even partial water retention within

the sample chamber will broaden the water transfer curve and reduce the mass 18 peak maximum. The implied lower water sensitivity for peak

height calculations indicates that the peak area method is to be

preferred.

6 . CONCLUSIONS

The ability to fill T05 packages with moist nitrogen at the 2000 and

5000 ppm levels has been demonstrated. The agreement between the water

content in the filling gas (1750-1690 and 4400-4200 ppm) agrees

remarkably well with the average measured content in the samples tested

(1850 and 4300 ppm). This may be partly fortuitous, but there was no discernable drift in water content throughout the procedure (120 capping

operations at about 3 minute intervals).

The package contents were not measurably altered over a 5 week interval

between testing. The precision of measurement was found to be within

6-8% at the low level and 3-5% at the high level. The difference between

the % deviations in Tables 3, 5, 6 and 7 give the precision of filling as 5-7% and 3% at low and high levels respectively.

The analytical procedure outlined above is designed to measure total

water content (i.e. adsorbed + gaseous) in small sealed volumes.

o # Prebaking at 100 C for intervals of 4 to 72 hours has not noticeably

affected similar previous measurements although at higher temperatures

(~300°C) water can be initially desorbed from the bulk of

electrolytically gold plated layers and eventually reduced by

dissociation at catalytically active metal surfaces. The more relevant

criticisms of the measurement procedure are as follows:

111
1. Large potentially adsorptive surface areas in the sample chamber

reduce the transfer rate of water to the detector. This places a limit

on the quantity of water detectable to about 500 ppm in a 0.1 m.bar litre

gas sample.

2. Sampling of the standard gas introduced an unlcnown quantity of water

which had been adsorbed on the wall of the calibration volume. This

quantity can be minimised by increasing the temperature of the

calibration volume and utilising more hydrophobic surfaces. The present

system is a viable working compromise but it does suggest that the larger

water content in the standard will result in a measured value in the

sample gas which is marginally lower than real value.

3. The effect of oxygen on the water sensitivity and the dependence of

that sensitivity on water content should be taken into consideration.

More extensive measurements over a longer time scale have indicated that

providing the system is hermetic and has been processed in a routine

manner, the precision of measurement does not change.

7 MODIFICATIONS IN PROGRESS

Several modifications are being made in order to minimise the problems

already discussed.

Calibration procedures for moisture content are now utilised in which the

partial pressures of water and oxygen in the standard gas are similar to

those which were found for the samples. A Michell 3000 hygrometer has

been installed to provide accurate dewpoint readings from +20 to

-30°C.

An improved sample chamber is also being constructed (Fig. 2). A much

larger stainless steel system with a carousel located and supported on ball bearings allows packages to be accomodated in 3" diameter sample

holders which are then sequentially presented to the analysis chamber

for puncture and gas transfer. Steel bellows and Viton 0-rings provide

the necessary vacuum seals. This unit will maintain the integrity of the

internal loading method with the following advantages over the old system:

112
a) Larger packages can be accommodated.

b) Interference effects from multiple packages within the same analysis

chamber will be avoided.

c) The analysis chamber will have a lower volume and a much lower and

consistent surface area.

d) Standard gases will be admitted from a heated calibration volume.

REFERENCES

1. Michell Instruments Ltd, Nuffield Road, Cambridge CB4 1SS, UK

2. NBSIR 84-2852, RADC/NBS Moisture Workshop, 1983, p 69.

3. Protimeter Ltd, Fieldhouse Lane, Marlow, Bucks SL7 1LX, UK.

4. J.A. Becker et al. J Appl Phys 32 ,p411 (1961)

5. L.J. Rigby, Can J. Phys 43, pl020 (1965)

ACKNOWLEDGEMENTS

The samples were prepared by J. McNicol and the analyses were carried out

by M. Kumar. The author thanks the EEC Community Bureau of Reference for

financial assistance and STL Ltd for permission to publish this paper.
File

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22

Table 1 T05 Low Level Test 1

Sample Protimeter Filling Gas volume Water % Water

Dew Pt/°C Dew point/°C /mbar . litre sensitivity content

AIR 12.0 0.0101 0.235

AIR 12.4 0.0101 0.224

30696 -16.90 0.177 0.053

DRY NITROGEN 0.101

AIR 12.4 0.0101 0.240

30698 -16.89 0.179 0.060

AIR 12.6 0.0101 0.239

30700 -16.85 0.171 0.040

AIR 12.8 0.0101 0.260

30702 -16.84 0.131 0.048

AIR 13.0 0.0101 0.285

30704 -16.84 0.174 « 0.046

AIR 14.0 0.0101 0.260

30705 -16.82 0.176 0.051

AIR 13.8 0.0101 0.262

30706 -16.83 0.174 0.048

AIR 14.0 0.0101 0.269

30708 -16.84 0.174 0.062

AIR 14.1 0.0101 0.254

30710 -16.80 0.155 0.069

AIR 13.6 0.0101 0.266

Average 0.254 0.053

Std. deviation 0.017 0.0086

% deviation 6.7 16

Data analysed 31st October, 1984

Atmosphere pressure 1012 mbar

114
Table 2 T05 Low Level Test 1

File Sample Protimeter Filling Gas volume Water % Water

No Dew Pt/°C Dew point/°C /mbar .litre sensitivity content

2 AIR 12.2 0.0101 0.229x

3 30716 -18.02 0.181 0.140

4 AIR 12.5 0.0101 0.286

5 30717 -17.60 0.177 0.163

6 AIR 13.0 0.0101 0.327

7 30718 -17.40 0.175 0.192

8 AIR 13.2 0.0101 0.276

9 30720 -17.20 0.134 0.181

10 AIR 13.5 0.0101 0.311

11 30721 -16.17 0.174 0.208

12 AIR 13.8 0.0101 0.316

13 30722 -15.46 0.177 0.195

14 AIR 13.9 0.0101 0.309

15 30724 -16.86 0.178 0.182

16 AIR 13.9 0.0101 0.288

17 30725 -16.90 0.180 0.179

18 AIR 13.9 0.0101 0.272

19 30726 -16.94 0.174 0.207

20 AIR 14.0 0.0101 0.275

21 30728 -16.72 0.177 0.175

22 AIR 14.0 0.0101 0.264

23 30729 -16.88 0.176 0.203

24 AIR 14.1 0.101 0.309

25 30730 -17.55 0.180 0.173

Average 0.294 0.183

Std. deviation 0.020 0.019

% deviation 6.8 10.7

x Omitted from average

Data analysed 2nd November, 1984

Atmosphere pressure 1012 mbar

115
File

No

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2

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13

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16

17

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20

21

22

23

24

25

Table 3 T05 Low Level 1

Sample Protimeter Deviation Gas volume/ Water

Dew Pt/°C /°C mbar . litre sensitivity

AIR 9.1 0.3 0.101 0.260

Background 9.3 0.4

AIR 9.5 0.6 0.0101

NITROGEN[ 9.5 0.6 0.100

AIR 9.6 0.7 0.0101 0.264

AIR 9.7 0.7 0.0101 0.239

30582 9.7 JO.

6

0.194

AIR 9.7 0.5 0.0101 0.234

30594 9.8 0.5 0.196

AIR 9.9 0.5 0.0101 0.239

30606 9.9 0.5 0.172

30617 10.0 0.6 0.171

AIR 10.1 1.2 0.0101 0.237

30631 9.3 0.3 0.165

30644 9.8 0.4 0.165

AIR 10.2 0.6 0.0101 0.255

30656 10.2 0.8 0.184

30668 10.5 0.8 0.182

AIR 10.5 0.7 0.0101 0.236

30680 10.4 0.6 0.167

30692 10.4 0.6 0.167

AIR 10.4 0.6 0.0101 0.284

30738 10.3 0.5 0.176

30751 10.4 0.6 0.174

AIR 10.4 0.7 0.0101 0.264

Average 0.251

Std. deviation 0.016

% deviation 6.3

Water content during filling = 0.176% (Michell dew point -16.

Data analysed

Atmosphere pressure

13th November, 1984

1008 mbar

% Water

content

0.211

0.137

0.151

0.181

0.237

0.182

0.172

0.189

0.185

0.174

0.205

0.197

0.185

0.025

13.5

116
Table 4 T05 High Level (Test)

File Sample Protimeter Filling Gas volume/ Water % Water

No Dew Pt/°C Dew point/°C mbar. litre sensitivity content

3 AIR 9.2 0.100 0.306x

4 30773 1.79 0.218 0.85

3 AIR 9.6 0.100 0.364

6 30774 0.77 0.219 0.94

7 AIR 9.9 0.100 0.362

8 30775 1.88 0.210 0.96

9 AIR 10.2 0.100 0.367

IP 30776 2.06 0.210 0.11

11 AIR 10.4 0.100 0.383

12 30777 1.86 0.233 1.02

13 AIR 10.8 0.100 0.367

14 30778 1.78 0.213 0.12

15 AIR 11.0 0.100 0.379

16 30783 -1.65 0.210 0.81

17 AIR 11.4 0.100 0.379

18 30784 -3.84 0.219 0.77

19 AIR 11.3 0.100 0.384

20 30782 -1.42 0.222 0.78

21 AIR 11.5 0.000 0.361

22 30781 1.98 0.214 0.78

23 30780 -1.40 0.213 0.78

24 30779 -0.98 0.213 0.84

25 AIR 12.1 0.100 0.393

Average 0.375 1.00 0.80

Std. deviation 0.011 0.085 0.024

% deviation 2.9 8.5 3.0

Cans were filled in numbered sequence at 10 min. intervals but with 40 mins,

between 30778 and 30779.

x Omitted from average

Data analysed 7th November, 1984

Atmosphere pressure 1000 mbar

117
Table 5 T05 High Level 1

File Sample Protimeter Deviation Gas volume/ Water % Water

No Dew Pt/°C /°C mbar. litre sensitivity content

1 AIR 9.5 0.4 0.10 0 . 252 x

2 NITROGEN 9.8 0.4 0.10

3 AIR 9.9 0.4 0.10 0 . 303 x

4 AIR 10.0 0.4 0.10 0.329

5 Background 10.1 0.4 0.10

6 AIR 10.2 0.4 0.10 0.329

7 30791 10.4 0.6 0.217 0.419

8 AIR 10.4 0.5 0.10 0.362

9 30794 10.4 0.5 0.209 0.439

10 30805 10.3 0.4 0.208 0.455

11 AIR 10.4 0.5 0.10 0.368

12 30816 10.5 0.6 0.217 0.418

13 30826 10.5 0.6 0.210 • 0.414

14 AIR 10.7 0.6 0.10 0.370

15 30836 10.6 0.5 0.213 0.415

16 30846 10.7 0.5 0.215 0.407

17 AIR 10.8 0.7 0.10 0.370

18 30856 10.8 0.6 0.215 0.460

19 30866 10.8 0.5 0.215 0.427

20 AIR 10.7 0.3 0.10 0.371

21 30876 11.0 0.6 0.216 0.423

22 30886 10.9 0.5 0.212 0.429

23 AIR 11.0 0.4 0.10 0.352

24 30896 11.1 0.5 0.213 0.432

25 AIR 11.2 0.6 0.10 0.381

Average 0.359 0.428

Std. deviation 0.018 0.015

% deviation 4.9 3.5

Water content during filling = 0.44% (Michell dew point -16.2)

Data analysed 14th November, 1984

Atmosphere pressure 1002 mbar

x Omitted from average

118
File

No

4

56

78

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

Table 6 T05 Low Level 2

Sample Protimeter Deviation Gas volume/ Water % Water

Dew Pt/°C /°C mbar. litre sensitivity content

AIR 3.0 1.1 0.101 0 ,170x AIR

30583

3.2 1.3 0.101

0.200

0.173 x 0.165

AIR

30595

3.5 1.1 0.0101

0.197

0.167 x 0.138

AIR

30607

3.9 1.1 0.0101

0.199

0.201

0.174

AIR

30618

4.0 1.0 0.0101

0.195

0.226

0.155

AIR

30633

4.4 1.2 0.0101

0.197

0.220

0.194

AIR

30645

4.9 1.2 0.0101

0.197

0.237

0.152

AIR

30657

4.3 0.8 0.0101

0.194

0.181

0.192

AIR

30669

4.5 1.1 0.0101

0.194

0.216

0.192

AIR

30681

4.3 1.1 0.0101

0.193

0.202

0.211

AIR

30693

4.3 1.1 0.0101

0.195

0.183

0.195

AIR

30739

4.0 0.9 0.0101

0.191

0.214

0.200

AIR

30752

4.1 1.1 0.0101

0.194

0.209

0.207

AIR 3.8 0.7 0.0101 0.205

Average 0.209 0.181

Std. deviation 0.016 0.023

% deviation 7.7 13

x Omitted from average

Data analysed 18th December, 1984

Atmosphere pressure 1008 mbar

119
File

No

2

3

4

56

78

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

Table 7 T05 High Level 2

Sample Protimeter Deviation Gas volume/ Water

Dew Pt/°C /°C mbar .litre sensitivity

AIR 10.9 0.6 0.101 0 . 311x AIR 11.0 0.7 0.101 0.321x 30793 0.211

AIR 11.0 0.3 0.101 0.335

30804 0.228

AIR 11.4 0.7 0.101 0.321

30815 0.218

AIR 11.3 0.6 0.101 0.310

30825 0.222

AIR 11.2 0.5 0.101 0.317

30835 0.215

AIR 11.1 0.4 0.101 0.324

30845 0.221 * AIR 11.4 0.5 0.101 0.301

30855 0.219

AIR 11.7 0.5 0.101 0.323

30865 0.215

AIR 11.8 0.5 0.101 0.337

30875 0.217

AIR 11.7 0.6 0.101 0.325

30885 0.222

AIR 11.5 0.4 0.101 0.317

30895 0.215

AIR 11.5 0.5 0.101 0.331

30906 0.215

AIR 11.4 0.6 0.101 0.311

Average 0.320

Std. deviation 0.0097

% deviation 3.04

x Omitted from average

Data analysed 20th December, 1984

Atmosphere pressure 1010 mbar

% Water

content

0.418

0.468

0.420

0.412

0.420

0.409

0.426

0.396

0.426

0.374

0.401

0.388

0.413

0.023

5.7

120
Table

8.

Typical

moisture

data

for

each

type

of

sample.

121
Enclosure

t

Dry N2

Fig.1 Encapsulation System

122
Baratron

Needle

valve

Carousel

rotation

Breaker

unit Std. gas

Jr I Tap

Mass

spec.

Viewing

and

loading

port

t

Gas pressured

ram

II

Turbo

pump

Sample

holder

and

thrust

guide

Fig.2 Schematic Diagram of External Carousel

System

123
©1986 IEEE, Reprinted with permission from the 36th Electronic

Component Conference, May 5-7, Seattle, Washington.

3.1 NEW FAILURE MECHANISMS IN TEMPERATURE AND HUMIDITY STRESS

D. Y. Guan* ,T. F. Gukelberger, E. C. Cahoon,

T. W. Joseph, and J. M. Snowdon

International Business Machine Corporation

Dept. A28, Bldg. 908

P. 0. Box 950

Poughkeepsie, New York 12602

Abstract

New failure mechanisms other than metal corrosion were observed in high-temperature, high-humidity testing of plastic encapsulated VLSI modules. The mechanisms were unique in that physical defects such as microcracks were associated with the failures. Fail- ures appeared as single-bit, column, or row failures

depending on circuit designs and process technology.

Additionally, for one process design, ionic accumulation contributed to the failures. Electrical diagnosis, physical analysis, and failure kinetics along

with the uniqueness of temperature and humidity (T/H)

testing to the mechanisms will be discussed.

Introducti on Temperature/humidity stress testing is performed

to assess the moisture resistance of plastic

encapsulated integrated circuit (IC) modules. The most extensively reported failure mechanism to date is electrolytic corrosion of the chip metallization. With

the advent of VLSI technology, however, increased circuit sensitivity has resulted in other T/H failure

mechanisms. These have been attributed to the conductive or capacitive effects of the build-up of a layer of moisture on the surface of the chip [1-4].

However, this paper will describe new failure mechanisms characterized by physical defects in the interlevel dielectrics. Both leakage-induced and

open-circuit failures have been observed.

Experiment

The test vehicles were vendor dynamic RAM (DRAM)

chips encapsulated in plastic dual-in-line packages.

Out of many vendors tested, two in particular, referred

to as Vendors A and B, had extensive' fails, and,

therefore, will be discussed in detail.

Multiple T/H conditions (85°C/85%, 85°C/81%,

85°C/75%, 70°C/85%) were used in this evaluation. During the first 168 hours in the stress chamber no bias was applied. This allowed the modules to achieve equilibrium with the stress environment while exposing

those "defect" fails which occurred in the absence of bias. After electrical readout the samples were re- turned to stress, with bias applied for 24 hours and

subsequently removed again for readout. This stress methodology provided a means of separating the "defect"

and intrinsic failure populations.

Conventional T/H tests were conducted for both

Vendor A and Vendor B. Additionally, Vendor A samples were tested under Highly Accelerated Stress Test (HAST)

conditions. These accelerated stresses (138°C/85%,

138°C/75%, 138°C/50%, and 127°C/85%) provided kinetic

data in a relatively short stress time [5].

ResultsMost of the failures for Vendor A came from HAST

stress and were predominantly row failures. Some single-bit failures were also observed. Physical

analysis showed that the row failures were due to cracks in the word line and interlevel oxide at a step over a first level polysilicon edge (see Figure 1).

The single-bit failures were due to a crack in the thin

oxide under the transfer gate of the failing cell as shown in Figure 2. In both cases the cracks were isolated to the areas around the fail sites and no cracks

were observed in the passivation above the fail sites.

Figure 1. Vendor A failing word line showing crack in refractory metal silicide line and interlevel oxide. SEM micrograph, 20,000x.

* Present address:

tion, NY 12533.

IBM Corp . , Rt. 52, Hopewell June- 124

Figure 2. Vendor A failing bit showing area where

transfer gate disappeared due to cracked thin oxide.

Optical photograph, 2000x.
Since the failures for Vendor A were due to cracks

causing added resistance or opens, they were not sen- sitive to test voltage or recoverable by baking at 150°C for 24 hrs. Single-bit failures were the predominant failure

mode for Vendor B's product subjected to T/H stress -with bias. The failures were found to be voltage, timing,. and pattern sensitive. Failures only occurred

during long-cycle, low V^, and disturbed patterns.

Electrical diagnostics indicated that the failures were

due to leakage between the cell and the bit line or cell to cell. These failures demonstrated voltage sensitivity in that higher Vcc could make the failure recover. Physical analysis found a crack in the passivation and

Interlevel oxide along the edge of a metal line running

over the failing cell (see Figures 3 and 4). One single

column fail was also observed. In this case, a crack

was found in the active restore circuitry, resulting

In leakage between bit line and V . These microcracks

could not be seen until after a light glass etch, due

to their narrow dimension and the severe chip topology.

,The cracks always extended down to one of the

.polysilicon layers and in some cases, to thp Si sur-

•face.

Figure 3. Vendor B failing bit showing crack in interlevel oxide over cell plate and transfer gate.

Optical micrograph, 2000x.

Figure 4. Vendor B failing bit showing crack in passivation glass along metal edge. SEM micrograph,

5000x

.

Subjecting Vendor B single-bit failures to a bake often produced recovery. Physical analysis of recovered single-bit failures always revealed a microcrack at the fail site, indicating that the crack alone was not enough to cause failure, and that moisture

and/or ionic build-up were necessary for the leakage

,to occur.

‘Kinetic Study

The failure population of Vendor B was bi-modal. One group of fails occurred early in stress (with and

without bias), whereas the second group, which required

bias , failed later In stress. Statistical analysis

of the data clearly indicated two populations, that Is, defect, and Intrinsic. The latter population fit well

to a lognormal distribution as shown in Figure 5. For Vendor A. no defect population was observed. Analysis

of the data, T/H and HAST likewise indicated a lognormal fit as shown in Figure 6. The activation energy a'JH was estimated to be 0.7

eV for Vendor A (Figure 7) and 1.0 eV for Vendor B. Recall that the failure modes were correspondingly

different, that is, single-bit failures (short) for

Vendor B; row fails (open) for Vendor A. Analysis of

the data is consistent with the following dependency

for the mean time to failure:

t

5Q Mexp ( B| / RH + AH / kT ) (1)

where, RH = relative humidity in fraction

T = absolute temperature in Kelvin

k = Boltzmann constant

then the humidity dependency value, B, is estimated as

1 for Vendor A and Vendor B (see Figure 8). This RH dependency is supported by the absence of intrinsic

failures in high-temperature bias (HTB) stress for both

Vendors A and B within a reasonable time.

Given the strong temperature dependence and the occurrence of failures relatively late in stress, the

intrinsic failure mode posed no field reliability concern f nr either vendor.

Pi scussi on The uniqueness of this study is the finding of

microcracks at the fail sites of modules subjected to T/H stress. If cracks are initially present in the

passivation, as in Vendor B, failure can occur during

stress at the location of the defect. Alternatively,

cracking could be induced by T/H/B stress alone.

Swelling of the plastic and/or organic layers through

moisture absorption could- lead to qrack generation anc propagation in the areas of high stress [6,7]. Due to high cell densities, DRAM technologies often employ narrow structures in multiple layers resulting in se- vere topology. This topology produces both horizontal

and vertical stresses in and between layers. Another

source of stress in DRAM technology is refractory metal

silicide lines. These silicides are an attractive alternative to polysilicon due to their low resistivity,

however, high formation temperatures result in high

2 tensile stress on the order of 10E9 dynes/cm [8].

In Vendor A, stress in the refractory metal

silicide line caused a crack in both the line and the # underlying oxide. This happened at a very steep 2000A

step, directly under the edge of a metal line, con- tributing to localized stresses. These cracks occurred

only after tests subjecting the parts to high humidity,

indicating that the increased stress due to swelling
Figure 5. Vendor B stress data.

Figure 6. Vendor A stress data.

caused the lines to crack, resulting in failures. None

of these failures were bake recoverable as the cracks

caused high resistance or opens.

In the case of Vendor B, during THB stress, cracks

occurred in passivation layers between dynamically

isolated nodes. Failure was due to leakage between

these nodes. The microcracks were always found in the area of the most severe topology along the edge of a metal line and over two layers of polysilicon. The fact

that a defect population was observed in Vendor B tends

to indicate that the stress alone in this structure was enough to cause a crack. Voltage was required to induce

failure during T/H stress indicating the possibility

of ionic accumulation in the crack [9]. These failures

were extremely sensitive to voltage, temperature, and

extended timings when being tested. The fact that most

of these fails were recoverable after baking may depend

on the size of the crack and the amount of ionic accu- mulation.

For both vendors the microcracks were found to be along a metal edge, but the orientation of the metal

line with respect to the cell was 90 degrees different

for the two vendors, leading to the different failure

modes

.

t^g depends strongly on temperature and only

weakly on humidity as indicated by small value of 3 in Equation (1). However, the presence of moisture is necessary for failure to occur as no intrinsic failures

occurred on HTB and temperature cycling tests. The

results of this study also indicate that moisture

transport through the package was not the rate deterFigure 8. Beta determination.

mining factor for failure as implied by the relatively

higher activation energy [10-12].

The Vendor B population defect failures occurred

early in stress, indicating that T/H testing is an effective way to screen out this failure mode. An equi

librium "soak" period, coupled with a frequent early

readout schedule, provided for a clear separation of the defect and intrinsic populations.

Conclusion

New failure mechanisms other than metal corrosion

and migration were found in THB testing of plastic

encapsulated VLSI modules. The fails appear as single

bit, partial column, or partial row failures depending

on circuit designs, defect locations, and process technology. Microcracks induced by T/H stress with and

without subsequent ionic accumulation were found to be two major causes for the failures. The high stress areas such as silicide lines and severe topological

points are more susceptible to cracking. Additional

stress from moisture absorption during T/H stress seems

to be critical to crack generation and propagation.

The defect could also be present before stressing, in which case the failure would occur after exposure to T/H, without bias, as observed.

The kinetic study indicated that the intrinsic

population followed lognormal distribution. The data

indicated a strong t^g dependence on temperature with

126
an activation energy of 1 eV for one vendor and 0.7 eV

for the other. The humidity dependency was found to

i be relatively small, however failure only occurred when

moisture was present.

Finally, no field reliability problem was

: projected with the intrinsic mode for either of the

vendor products tested.

Acknowledgements

The authors wish to thank J. Gunn for providing HAST,

P. Tasi for his excellent failure analysis, and G. Dauser and R. Beilin for electrical diagnostics.

References

1. S. Shabde, J. Edwards, and W. Meuli, "Moisture In- duced Failure Mode in a Plastic Encapsulated Dynamic Timing Circuit," Proc. Rel . Phys. (1977)

33.

2. H. Gunther, H. Muller, J. R. Goetz, and D. Kantz,

"Moisture Induced Failure in Plastic Encapsulated

Dynamic RAMs," Proc. Electronic Components Confference, (1983), 344.

3. D. Stoehle, "Influence of the Chip Excess Temperature on the Moisture Induced Failure Rate of Plastic Encapsulated Devices," Proc. Electronic Comp- onent Conference, (1983), 253.

4. M. Noyuri , T. Ishihara, and H. Higuchi, "Secondary

Slow Trapping - A New Moisture Induced Instability

Phenomenon in Scaled CMOS Devices," Proc. Rel. Phys.,

(1982), 113.

5. J. E. Gunn, R. E. Camenga and S. Malik, "Rapid Assessment of the Humidity Dependence of IC Failure

Modes by Use of HAST," Proc. Rel. Phys., (1983)

66 .

6. N. Nagasima, H. Suzuki, K. Tanaka and S. Nishlda,

"Interaction Between Phosphosi 1 icate Glass Films and

Water," J. Electrochemical Society: Solid Science

and Technology Vol . 121, No. 3, (1974), 434.

7. R. F. Fedors, "Cracking in a Glassy Epoxy Resin

Induced by Water Absorption," Polymer, Vol. 21,

(1980), 713.

8. S. P. Murarka, Silicides for VLSI Applications,

Academic Press, NY, (1983) p.60.

9. K. Watanable, T. Tanigaki, and S. Wakayama, "The

Properties of LPCVD Si^ Film Deposited by S^C^

and N£q Mixtures," J. Electrochemical Society: Solid

State Science and Technology, Vol. 128, No. 12 (1981), 2630.

10. R. M. Levin, "Water Absorption and Densification

of Phosphosi 1 icate Glass Films," J. Electrochemical

Society: Solid State Science and Technology, Vol.

129, No. 9,(1982), 1765.

11. A. Apicella, L. Nicolais, G. Astarita, and

E. Drioli, "Hygrothermal History Dependence of

Moisture Sorption Kinetics in Epoxy Resins,"

Polymer Eng, and Science, Vol. 21, No. 1, (1981),

18.

12.

E.J. Mclnerney and P. A. FI inn, "Diffusivity of

Moisture in Thin Films," Proc. Rel. Phys. (1982),

265.

127
3.2 STUDIES OF MOISTURE IN POLYIMIDE: A SUMMARY

Stephen D. Senturia

Microsystems Technology Laboratories

Department of Electrical Engineering and Computer Science

Masachusetts Institute of Technology

Cambridge MA 02139

(617) 253-6869

Polyimide is finding increased use in microelectronic devices as a passivant, interlevel insulator,

and die-attach adhesive, in part because of its excellent electrical properties, chemical, mechanical, and

thermal stability, ease of application, and its good planarization and etch properties [1]. Like all polymers,

polyimide absorbs moisture, and because of the link between moisture absorption and potential reliability

problems, the role of polyimide's moisture absorption in ultimate device reliability must be carefully as- sessed. On the one hand, published data indicate improved reliability if polyimide is used in conjunction

with inorganic passivants, when compared with the inorganic passivants alone. Furthermore, the fact that polyimide does absorb moisture provides a degreee of moisture gettering in finite-volume hermetic packages, even in cases where total moisture content of the package might exceed the recommended minimum. On the other hand, the role of moisture in degrading polyimide adhesion is well documented, and

in the event of adhesion loss, moisture absorbed in the polyimide could collect in unadhered regions,

creating the potential for corrosion.

This paper summarizes the results of a series of our previous studies [2-11] of moisture uptake

and diffusion in polyimide films in integrated circuit structures, and of the effects of moisture on polyimide

properties. Because of the large number of literature citations contained in these papers (more than 80),

they will not be repeated or discussed here. The reader is directed especially to References 3-5, 7, 8, and

1 1 both for the citations and for a discussion of the relation between our findings and other published

work.

We have carried out both gravimetric [2-4, 6] and electrical [2-6,8] studies of moisture absorption.

Sample structures are illustrated in Figures 1 and 2. The simplest structure is a polyimide film spin-cast

128
onto an aluminum-coated silicon wafer (Fig. la). In this case, the path for moisture uptake is normal to the

surface of the as-deposited film; we call this a "vertical" sample. Alternatively, one can investigate samples

in which a metal film is photolithographically patterned on top of the polyimide (Fig. 1b). If the polyimide is etched away as shown in Fig. 1b, the path for moisture uptake is parallel to the polyimide surface; we call

this a "lateral" sample. Samples of both types have been used for gravimetric studies [4,6]. If the polyimide is not etched away, the moisture uptake path to the region beneath the metal stripes involves a combination of vertical and lateral diffusion. Much of our early electrical work [2,3,5] was done with unetched

samples, and was analyzed assuming isotropic diffusion characteristics. We subsequently discovered that the kinetics of moisture uptake differed for vertical and lateral samples [6]. As a result, there remains a small

quantitative uncertainty in the detailed intepretation of some of the electrical data from unetched samples,

but this does not affect the general properties as described here.

Sample fabrication begins with evaporation of 1 jim of aluminum onto a cleaned silicon wafer.

This aluminum layer is used even for the gravimetric samples so that rigorous comparison with the electrical samples, which require a conducting lower electrode, can be made. The polyimide is then spin deposited

and cured. We have examined three basic polyimide chemistries, PMDA-ODA, BTDA-MPDA/ODA, and a

fluorinated modification of the BTDA material. While there are some quantitative differences among the

various polyimides, they are not the principal subject of our research; hence we report here primarily on

the widely-used PMDA-ODA material. The polyimides were obtained from Dupont and IBM in the form of a polyamic acid precursor dissolved in NMP. Samples of various thickness are achieved either by dilution of the precursor in NMP, or by multiple coats. Following spinning of each coat, the film is partially imidized at a temperature of 180°C. After all coats have been applied, the film is cured in nitrogen at a final cure temperature which was varied between 250 and 400°C. Thicknesses in the range 2 - 15 pm have been studied,

although most studies were for either 3 pm or 6 pm films. For samples with an upper metal layer, 1 pm of aluminum is evaporated after polyimide cure, and is patterned photolithographically to achieve a multiplestripe electrode (Figure 2). Etching of the polyimide is done after aluminum patterning in an oxygen

plasma, with the alumimum electrode serving as the etch mask.

Another approach to sample fabrication was to develop a complete polyimide test chip containing a variety of test devices [8], including parallel-plate and interdigital capacitor structures specifically designed for the moisture studies. While this approach requires a much more elaborate mask set and pro- cess specification (because some of the measurement techniques involve MOS transistors), it has the

advantage of permitting tightly correlated in-situ measurements of a wide variety of properties (ion transport, conduction, and moisture uptake) on the same polyimide sample.

129
We now review experimental findings. Figure 3 shows a typical gravimetric moisture-uptake transient, plotted as mass change against time starting from a point at which the ambient moisture is suddenly

increased. To make these measurements, the coated wafer is hung in a Cahn 1000 microbalance, tared

against a similar uncoated wafer. The moisture ambient of the test wafer is varied by control of the dew

point of an air stream which enters the microbalance chamber through a baffle above the wafer position

and exits from the same side of the microbalance below the wafer position. The dew point is monitored

with a General Eastern dew-point hygrometer. By piecing together mass-change transients like those of Figure 3, it is possible to determine the total mass change with relative humidity over the entire range to about 85% RFI. Above 85% RH, surface condensation appears to affect the microbalance measurement.

Below 85% RFI, the moisture uptake is proportional to sample thickness, hence is a bulk effect, and is approximately linear in RH. By extrapolating to 100%, we find a total moisture uptake that varies from 2 - 3%.

Interestingly, we usually find a higher moisture uptake than that reported for free-standing bulk samples. A

possible explanation is that the as-deposited films are under tensile stress [9-11], which can increase the

moisture absorption. From the shape of the moisture-uptake transient, it is also possible to extract the

diffusion constant based on a Fickian model. These results are discussed later. Moisture uptake can also be studied by measuring the change in capacitance of a parallel-plate

[2-6, 8] or interdigital [5,8] capacitor in which the polyimide serves as the dielectric. Figure 4 shows typical

normalized capacitance vs. RH for a parallel-plate capacitor structure with a stripe-electrode upper plate.

The data include room temperature points for which the dew point of the ambient air was varied, and points

for temperatures in the range of 20 - 80°C measured with fixed-dew-point air. Both sets of data are consistent with the basic observation that the dielectric constant of the polyimide varies approximately

linearly with relative humidity.

The correlation between the gravimetric and dielectric measurements of moisture uptake permit

the extraction of the effective dipole moment of the absorbed water molecule [4]. The molecular polarizability of the absorbed water is extracted from the change in dielectric constant using the Clausius-Mossotti

relation, and is plotted in Figure 5 against the concentration of absorbed moisture molecules, obtained

from the gravimetric data. The solid curves are calculated for various values of dipole moment. It is seen

that the absorbed water molecule has a moment equal to about 0.9 of that of the free water molecule (p0

). This indicates that the absorbed moisture is extemely weakly bound, possibly residing in microvoids that

result from the removal of water during the condensation cure reaction which converts the polyamic acid

precursor into the polyimide. This conclusion is supported by a decrease both in moisture absorption and

diffusion constant with increased cure of the polyimide film [5].

130
Analysis of the moisture-uptake transients with a linear Fickian model can be used to extract the

diffusion constant and the surface equilibration rate. In the earliest work [3], it was assumed that the diffusion was effectively one-dimensional, and that the equilibration of the film surface with the change in ambient moisture was instantaneous. Figure 6 shows an Arrhenius plot of the resulting moisture diffusion

constant against temperature. The diffusion constant is on the order of 5 x 1 0"9 cm2/sec at room temperature, and is weakly temperature dependent, having an activation energy of about 0.3 eV. We also find

that the diffusion constant is 35% greater for absorption than for desorption at room temperature; this asymmetry, however, decreases with increasing temperature, approaching equal absorption and desorption rates at 100°C (see Figure 7). These values may have to be modified slightly by our more recent dis- covery [6] that a better model for the moisture transient is the so-called "evaporation" boundary condition,

and, further, that the surface equilibration rate which enters this boundary condition is about a factor of 9

faster for the sidewall of the lateral sample than for the as-deposited surface of the vertical sample. We also

find that the diffusion constant is 50% greater in lateral samples. Further studies are in progress to determine whether this difference is due to the presence of an etched sidewall, to an intrinsic anisotropy in the

diffusion characteristics of the film, or to enhanced diffusion at the metal-polyimide interface.

The effects of moisture on electrical conduction in polyimide aredifficult to document quantitatively, because when a voltage is applied to a sample, the resulting current exhibits very long-lived transients

at ordinary device temperatures. We denote the shortest transient, which typically exhibits a power-law

decay with time, as the "polarization" current, and the value of current that results after settling of the polarization current as the "transport" current, although both currents may in fact be transients on the time scale

of actual device use. Figure 8 illustrates how moisture increases both components of the current [7]. However, the increase is not considered to be large enough to pose a significant reliability problem as long as the polyimide remains well-adhered to the substrate. Hence, the critical issue is maintenance of adhesion, which, in turn, may be affected by residual stress. We have recently developed a suspended-membrane technology using microfabrication methods to create test site with which the residual stress, modulus, adhesion, and elongation at break of thin polymer films can be studied [9-11]. Figure 9 illustrates the simplest type of sample in schematic form. To

make the sample, a diaphragm is first etched in a silicon wafer using micromachining techniques; then the

polyimide is spin-deposited and cured; finally, the diaphragm is etched away leaving a free-standing membrane. To date, the principal use of these structures for moisture studies has been the demonstration of the well-known result that moisture exposure severely the adhesion of polyimide to silicon dioxide. However, we plan a more detailed study of this adhesion degradation by combining in-situ measurements of

residual stress and modulus in conjunction with the peel adhesion experiment.

131
ACKNOWLEDGEMENT

This work was support in part by IBM, E. Fishkill, NY, and in part by E. I. DuPont de Nemours &

Company, Wilmington, DE. The author is indebted to Denice D. Denton for her assistance in providing a

critical reading of the manuscript.

REFERENCES

[1] -For a review of recent literature, see "Polyimide and VLSI: A Research Perspective", S. D.

Senturia, R. A. Miller, D. D. Denton, F. W. Smith, III, and H. J. Neuhaus, Proc. Second Inti. Conf. on Polvimides . Ellenville, NY, October 1985, pp. 107-118, and additional review articles cited

therein.

[2] "Moisture Transport in Polyimide Films: Implications for Moisture Measurement in 1C Packages",

D. D. Denton, D. R. Day, and S. D. Senturia, Proc. RADC/NBS Workshop on Moisture

Measurement and Control for Semiconductor Devices III . Gaithersburg, MD, November 1983;

NBS Report NBSIR 84-2852, pp. 279-287.

[3] "Moisture Diffusion in Polyimide Films in Integrated Circuits", D. D. Denton, D. R. Day, D. F. Priore,

S. D. Senturia, E. S. Anolick, and D. Scheider, Journal of Electronic Materials . 14, 119-136

(1985).

[4] "Effects of Moisture Uptake on the Dielectric Permittivity of Polyimide Films", D. D. Denton, J. B. Camou, and S. D. Senturia, Proc. ISA Moisture and Humidity Conference . Washington, DC, April

1985, pp. 505-513.

[5] "Fundamental Issues in the Design of Polymeric Capacitive Moisture Sensors" , D. D. Denton, S. D. Senturia, E. S. Anolick, and D. Scheider, Proc. 3rd International Conf. on Solid-State

Transducers . Philadelphia, June 1985, pp. 202-205.

[6] "Vertical and Lateral Moisture Transport in Polyimide Films", D. D. Denton, J. B. Camou, and S. D. Senturia, Electronic Materials Conference, Boulder, CO, June 1985; manuscript in preparation

for Journal of Electronic Materials.

[7] "Conduction in Polyimide Between 20 and 300 C", F. W. Smith, III, Z. Feit, D. R. Day, and T. J. Lewis, H. J. Neuhaus, and S. D. Senturia, Electronic Materials Conference, Boulder, CO, June

1985; Journal of Electronic Materials , in press.

[8] "An Integrated MOS Test Chip for Study of the Electrical Properties of Polyimide Films", D. D. Denton, D. F. Priore, H. J. Neuhaus, S. D. Senturia, E. S. Anolick, and D. Scheider, Proc. Second

Inti. Conf. on Polvimides . Ellenville, NY, October 1985, pp. 521-528.

132
[9] "The Use of Micromachined Structures for the Measurement of Mechanical Properties and

Adhesion of Thin Films", M. Mehregany, M. G. Allen, and S. D. Senturia, Proc. IEEE Workshop on

Solid-State Sensors . Hilton Head, S. C., June 1986.

[10] "Novel Microstructures for the Study of Residual Stress in Polyimide Films", M. Mehregany, M. G.

Allen, R. T. Howe, and S. D. Senturia, Electronic Materials Conference, Amherst, MA, June

1986.

[1 1 ] "Microfabricated Structures for the In-Situ Measurement of Residual Stress, Young's Modulus,

and Ultimate Strain of Thin Films", Mark G. Allen, Mehran Mehregany, Roger T. Howe, and

Stephen D. Senturia, submitted to Applied Physics Letters .

133
Figure 1 Schematic cross sections of (a) vertical and (b) lateral moisture-uptake samples.

102/i.m

L

Figure 2 Schematic top view of a lateral moisture-uptake sample.

Figure 3 Typical gravimetric moisture-uptake

transient. MA

and Mg are the steady

state values before and after the

change in ambient humidity.

134
Figure 4 Steady-state normalized capacitance

vs. relative humidity.

Figure 5 Correlation of capacitance and

gravimetric moisture-uptake data.

135
Figure 6 Arrhenius plot of diffusion constant.

ARH (%)

84.5 34.4 12.7 5.4

Figure 7 Ratio of diffusion constant (D-»W)

measured for absorption to that

measured for desorption (W-»D).

136
10

‘8 r10 -9 - & 10 -’ 0 -

E

3 10 -n _

c

o>t 10'12 - u

g^io-13 -

10'14 -

10-15

•X

'°o

PMDA-ODA

Charging -

T = 50°C

V = 35V _|

t

= 3.3/im

°o Oo oo °14%RH

> *<i%rh

10'1 10° 101 10 2 103 104

10 5 log time (sec)

Figure 8 Effect of moisture on polarization

power-law region) and transport

current.

Suspended membrane test structure

Load-deflection measurements

Figure 9 Schematic cross-section of membrane

used for mechanical property and

adhesion (end-point) studies.

137
Reprinted with permission from RCA Review, Vol 45, June 1984.

3.3 MOISTURE CONTROL IN HERMETIC LEADLESS CHIP CARRIERS

WITH SILVER EPOXY DIE ATTACH ADHESIVE

D.R.Carley and R. W. Nearhoof, RCA Solid State Divion, Somerville ,N.J. 08876

R. Denning, RCA Laboratories, Princeton, N.J. 08540

ABSTRACT

This paper describes package design and process changes in the development of

an improved system for controlling internal water vapor levels below 5000 ppmv

in hermetically-sealed leadless chip carriers with silver-epoxy die attached

silicon devices.

It is shown by mass spectrometry that the silver-epoxy die attach material

releases water and unreacted monomers at about 300°C.

The high-temperature braze seal design and process originally used in

manufacturing was changed to a controlled-environment lew-temperature

seam-welded design. A high-temperature preseal furnace bake-out and epoxy

cure process was also incorporated. Moisture levels measured for the new

package design and process have been below the 1000 ppmv level.

INTRODUCTION

An excessively high moisture level was detected on some product that was being

fabricated for a critical high-reliability, program. The product, CMOS/SOS

devices, was assembled in leadless chip carriers (LCC) using DuPont 6838

silver-filled epoxy die attach adhesive (1)

.

Over a period of three years, a number of engineering tests and experiments

were made, only those considered most significant are reported.

138
Initial production of four universal gate arrays (UGAs) and one random access

memory (RAM) , all CMOS/SOS devices assembled in leadless chip carriers,

commenced in the fourth quarter of 1980 at the Findlay, Ohio facility. The

ICC packages were designed for braze seal, using a nickel-plated, gold-plated

Kovar lid with a An/Sn brazing preform tacked to the lid at each of the four

comers. The assembled package and lid are pre-aligned in a jig and damped

together with spring tension clips. The clipped assembly is placed on a

conveyor belt and passed through the furnace in a nitrogen atmosphere (20 pprnv

moisture) at a maximum temperature of 310° - 315°C (Figure 1)

.

The initial water vapor measurements on three production lots sealed from

December 1980 to November 1981 ranged from 460 to 2250 pprnv (parts per million

by volume) . A maximum of 5000 pprnv moisture is allowable for qualification to

Mil-Std-883, Test Method 1018.2, Proceedure 1 (2). These results were

satisfactory and qualified production; however, moisture measurements made on

engineering tests processed during the March and April 1982 period of time

ranged from 5000 pprnv to 15000 pprnv. Further moisture measurements made on

the product, after the production line at Findlay, Ohio was carefully checked

and controlled, indicated some product with internal moisture levels greater

than 5000 pprnv. Figure 2 indicates the moisture level distribution of product

made during this time. An engineering investigation into the cause of these

inconsistencies was initiated.

ANALYTICAL TESTS

Empty Packages vs. Packages with Silver-Epoxy Die Attached Devices

One of the first series of tests run compared empty packages and packages

containing the silver-epoxy die-attached devices for moisture levels (Table

1) . The mass spectrometer results indicated a significant increase in CO2 for

the packages containing the silver-epoxy die attached CMOS/SOS devices. The

moisture levels, however, showed no significant differences. Hydrogen,

generally present in hermetic packages due to the nickel plating or gold

plating processes (3) , was significantly lower for the package containing the

silver-epoxy die-attached devices . 139
It was clearly evident that the organic silver-epoxy die attach material had

an influence on the gases contained in the leadless chip carrier hermetic

package. It was also evident that at the high temperature required to braze

seal, additional degassing or moisture release could occur (Figure 3)

.

ANALYTICAL MEASUREMENTS

Die-Attach Silver-Epoxy Mass Spectrometry Analysis (4)

A sample of cured silver-epoxy die-attach material was inserted into a mass

spectrometer ion source by means of a pyrolytic probe, and the mass

spectrometer was scanned from 10 to 600 a.m.u. once a second. The sample was

heated at 200°C per minute to 1100°C. In this manner, identification of the

evolved gases was made as the sample was heated. Two distinct regions were

observed (Figure 3) . At about 300°C, unreacted monomers of the epoxy

( epichlorohydrin and bisphenol A) were evolved, along with degradation

products from these species, in particular water (Figure 3) . These data

indicate that one source of moisture can come from the silver-epoxy die attach

material. It suggests that at the braze seal temperature of 310° - 315°C, the

water released from the silver epoxy could be captured within the package

cavity as the braze seal is made, resulting in packages with greater than 5000

ppmv moisture levels (5)

.

ENGINEERING TESTS

Pre-Seal High-Temperature Bake-Out and Epoxy Cure

A pre-seal furnace bake-out and epoxy cure step through the braze seal furnace

at 330°C was tested. This pre-seal high-temperature process was considered

advantageous, since it would compensate for any insufficiencies in the

pre-seal bake, would ensure the elimination of absorbed and condensed moisture

from the package components, and would further out-gas the epoxy die attach

material and release same of the unreacted chemicals and water shown to be

present from the mass spectrometry analysis (Figure 3) . This process could

also be easily implemented in production.

140
Although a significant improvement in the distribution of moisture vapor

content was made, this process did not consistently produce product below the

5000 ppmv of moisture required (Figure 4)

.

Si/Au/Sn Preform for Moisture Control

The incorporation of an Si/Au/Sn alloy in a hermetic package was reported to

reduce the high levels of moisture (6) . Preliminary tests were conducted by

merely dropping a 0.35% Si/Au/Sn preform (0.070 x 0.070 xO.001 inches) in the

package with a silver-epoxy mounted chip and braze seal with no other

precautions taken to reduce moisture. The mass spectrometer moisture analysis

results are shown in Table 2. For these tests, the moisture level for the

control units measured 13,000 - 16,000 ppmv of moisture; the test packages

containing the 0.35% Si/Au/Sn preforms measured only 125 - 135 ppmv moisture.

Also, the hydrogen content of the packages containing the Si/Au/Sn increased

to 5000 - 8000 ppmv, compared to the control packages with 1700 - 1900 ppmv

hydrogen. These results confirm the proposed mechanism of the reaction of the

Si in the preform during the heat cycle. Si + 2 H2O = SiC>2 + 2 H2 (6)

.

The use of a Si/Au/Sn preform inside a package with an epoxy-mounted die

provided the silicon for the reaction to produce a very dry package.

Unfortunately, the method of placing the Si/Au/Sn preform inside the package,

and control of the related processing were not compatible with production

processes

.

Neither the Si/Au/Sn preform nor a Si/Au preform could be used as a die attach

since the device substrate is sapphire. All of the Si/Au/Sn experiments

involved CMOS/SOS die and silver epoxy for die attach. The development of a process using the Si/Au/Sn approach to control moisture is considered

possible. However, the results of the next series of experiments proved more

satisfactory from a cost and implementation point of view, and work on the

Si/Au/Sn preform approach was discontinued.

141
Seam Weld Seal

A modification of the braze seal package is required for seam welding. The

top layer of ceramic is reduced in thickness to allow the weld ring to be

brazed on top of it ( Figures 5 and 6 ) . The gold plating is removed from the

lid and the lid thickness is reduced to insure seam welding quality. The

seam-weld sealing is controlled under dry box conditions of less than 20 ppmv

moisture and sealed at room temperature. Initial engineering weld-seal tests

on 48-pin packages with silver-epoxy die attach measured between 0-410 ppmv

for moisture. The first series of tests with 24-pin and 64-pin weld-seal

leadless packages compared the weld-seal process and pre-seal furnace bake

with the braze-seal process and pre-seal furnace bake (Figures 7 and 8)

.

Moisture analysis results are compared in Tables 3 and 4 for the 64-pin

leadless carriers. The results indicated a significant (10-20 times)

improvement in moisture for the seam-weld IC's. Moisture levels of 100-300

ppmv for seam-welded IC's were measured versus 1700-2200 ppmv for the

braze-seal product. Based on these results, production was re-started with

weld-seal packages. A total of 109 seam-welded IC's were measured for

moisture (Figure 9) ; 90% of the product was belcw 300 ppmv and 63% of the

product was below 100 ppmv moisture, a significant and dramatic improvement

over the pre-seal bake braze seal process (which had approximately 16% of the

product above the 5000 ppmv moisture levels) (Figure 4)

.

Seam-Weld Seals - Internal Moisture Analysis at High-Temperature

Stress Conditions

A series of experiments were conducted for the seam-welded IC's to test

changes in the ambient gases under temperature stress.

The first series of tests were to qualify the process, and tests were

conducted at the highest temperature the devices would be subjected to in the

application. Ten cycles from 25°C to 200°6 were used to simulate the solder

board mounting process. The 64-pin seam-welded packages containing

silver-epoxy die attached CMOS/SOS devices, pre-seal furnace baked at 330°C,

were analyzed for internal gases before and after the ten-cycle temperature

stress (Tables 3 and 5) . Moisture levels of 100-300 ppmv were obtained before

temperature stress, and of 100 - 385 ppmv were measured after stress,

indicating a stable system under this type of stress.
A series of tests were next conducted with 125°C, 230°C and 313°C post-seal

bakes for these 64-pin leadless seam-weld IC's (Table 6) . Moisture analysis

was also made for seam-weld IC's before and after 125°C, 11V, 240 hour bum-in

( Table 7)

.

Mass spectrometer results indicated that at temperatures above 200°C,

significant changes in the ambient gases resulted ( Table 6) . Moisture

increases from 70-80 ppmv moisture at 125°C to 5000 - 9000 pprav moisture at

313°C were observed. However no changes in internal moisture levels were

observed for the 125°C temperature stresses.

DISCUSSION

It is shown that at approximately 300°C (Figure 3) , the silver-epoxy die

attach material chemically reacts, or decomposes, releasing water as a

by-product of the reaction. It is this reaction and water release that makes

it difficult to consistently obtain internal moisture levels of less than 5000

ppmv for the high-temperature brazed-seal leadless chip carrier. The addition

of the pre-seal high-temperature degassing, desorption and cure bake process

had an advantageous effect on moisture control (Figure 4) , although levels

below 5000 ppmv were not consistently maintained.

The seam-weld sealing process yielded product with internal moisture readings

consistently below 1000 ppmv, and typically at 200 ppmv (Figure 9)

.

Weld-sealed silver-epoxy die attached IC's, when subjected to post-seal

temperatures above 200°C, show significant changes in the ambient gases, and

increases in moisture content in the package.

CONCLUSION

Silver-epoxy die attached devices hermetically packaged in ceramic leadless

chip carriers can be successfully controlled for internal moisture levels with

the incorporation of (1) a high temperature 330°C pre-seal bake and (2) the

conversion from a high-temperature braze seal technique to a low-temperature

seam-weld seal. These devices, when bumed-in for 240 hours at 125°C, show no

internal changes in moisture levels.

143
REFERENCES

1. E. I. DuPont DeNemours & Co. Inc., "Epoxy Bonding Adhesives for

Microelectronic Applications", E-10682, 8/76

2. Robert W. Thomas, "Test Method 1018.2 - A Progress Report", NBS/RADC

Workshop, Moisture Measurement Technology for Hermetic Semiconductor

.

Devices, II, NBS Special Publication 400-72, 4/82 Pages 126-127.

3. R. K. Lowry, "Gaseous Compositions of Hermetic Cavity Ambients", NBS/RADC

Workshop, Moisture Measurement Technology for Hermetic Semiconductor

Devices, II, NBS Special Publication 400-72, 4/82 pages 64-75.

4. J. Gale and B. Betz, "Epoxy Die Attach Analysis", Private Communications

February 1984

5. D.T. Somerville, "The Role of Hybrid Construction Techniques on Sealed

Moisture Levels", 15th Annual Proceedings Reliability

Physics, 1977, pages 107-111.

6. M.L„ White, K.M.Striny and R.E. Sammons, "Attaining Low Moisture in Hermetic

Packages", 20th Annual Proceedings Reliability Physics,

1982, pages 253-259.

7. A Czandema, R. Vasofsky and K. Czandema, "Mass Changes of

Adhesives Daring Curing", Proc. 1977 International Microelectronic

Symposium, October 1977, pages 197-208.

144
ACKNOWLEDGMENTS

The authors gratefully thank G. L. Schnable for his interest and critical

reading of this report. P. Jane Gale and B. L. Bentz of RCA, Princeton

laboratory for the analytical analysis; R. St. Amand and R. Erdman, for

internal moisture measurements; and J. Nielsen, L. Nemeth, J. Morgan and D.

Glovich for their technical support and assistance. We are also grateful to

R. Zeien, J. Patel, A. Shaw, C. Blew and P. Eugan, of the Solid State

Technology Center for their suggestions, discussions and interest in factors

affecting internal moisture.

We also wish to acknowledge the financial support provided by the United

States Air Force (AFAC) Aeronautical Division, Wright Patterson AFB, Ohio,

Contract F3 3 615-77-5158 for funding part of this engineering effort on

heremetic leadless chip carrier packaging methods.

145
Fig. 1 —Flow chart for braze-seal process with pre-seal vacuum bake.

146
o

o

147

*

SCALE

CHANGES

TO

IOOO

PPMV

PER

DIVISION

Fig.

2

—

Moisture-level

distribution

in

early

samples

of

devices

made

using

braze-seal

process.
TOTAL

ION

TRACE

148
40

2 30

hcn

idH

cn 20 zD

u.O

CO

LU

CD2

D 10 z

0

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120 DEVICES MEASURED

20 MEASUREMENTS EXCEED 5000 PPMV

R=0-10500 PPMV

X = 3000 PPMV

< 1% C02 LEVELS

IZ

2d

mm

V

V,

25 12"ZT TZEZ3

60bo 8000 ' 2000 10000

MOISTURE IN PPMV- MASS SPECTROMETER

Fig. 4—Moisture-levei distribution in sample of devices made using brazeseal process with pre-seal furnace bake.

149
64-PIN WELD-SEAL

48-PIN WELD-SEAL

Fig. 5 —Weld-seal and braze seal packages with and without lid attached.

The 64-pin packages measure 0.720 inch square and the 48-pin package

0.560 inch square.

150
3-

LAYER

CERAMIC

151
Fig. 7—Flow chart for braze-seal process with pre-seal furnace bake.

152
Fig. 8 —Flow chart for weld-seal process with pre-seal furnace bake.

153
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200

400

600

800

1000

1200

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Moisture-level

distribution

in

sample

of

devices

made

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weld-seal

process

with

pre-seal

furnace

bake
Additional Moisture Information

Figure 9A represents an updating of Figure 9 with 158 additional

samples tested. All 267 samples were measured by the same Lab.

156
]

i

Table 1 —IC Package Ambient Analysis (MIL STD 883, Method 1018, Proc. 1): Empty

Packages Versus Packages Containing Silver-Epoxy Attached Devices

Empty Packages Packages with Standard Devices

64-Pin Leadless

Moisture - ppmv 300 - 1100 100 - 1700

Hydrogen - ppmv 2000 - 4500 300 - 800

C02 — ppmv 30 - 200 4000 - 7000

24-Pin Leadless

Moisture - ppmv 400 - 2700 200 - 2000

Hydrogen - ppmv 100 - 3000 300 - 900

C02 — ppmv 200 - 300 3000 - 14,000

157
Table 2 —IC Package Ambient Analysis (MIL STD 883, Method 1018, Proc. 1): Control IC Package Versus Package with Si/Au/Sn Preform Insert

Control Si/Au/Sn Preforms

Moisture - ppmv 13,000 - 16,000 125 - 135

Hydrogen - ppmv 1,700 - 1,900 5,000 - 8,000

C02 - ppmv 18,000 - 22,000 19,000 - 25,000

158
Table

3—

Moisture

Level

Tests

for

64-Pin

Packages:

Weld-Seal

Control

Group,

BrazeSeal

Control

Group,

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Weld-Seal

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Table 4 —Moisture Analysis of Post-Seal Temperature Stressing 64-Pin Leadless

Seam-Weld Seal

Moisture Level

(ppmv)

120°C - 16 Hours 70 - 80

230°C - 3 Hours 800 - 1,600

313°C - Furnace Bake - 3 Min. 5,000 - 9,000

160
3.4 POLYMERIC MATERIALS FOR MOISTURE PROTECTION OF MEDICAL IMPLANTS

Philip R. Troyk

Pritzker Institute of Medical Engineering

Illinois Institute of Technology

IIT Center, Chicago, Illinois, 60616

ABSTRACT

Within the past few years a number of devices known as "Smart Sensors" have

been developed. These thin-film devices integrate a sensing element and

processing electronics on the same die. Typically these sensors are not

suitable for presently used methods of hermetic packaging such as welded cans

due to their sub-miniature size, and need for exposure of the die to the sensed

environment. Organic coatings have been considered as alternative packaging

for these devices. The requirements for such coating materials depend upon the

environment in which the sensor must operate, as well the method of lead

attach. Techniques for interface from a flexible multi-conductor lead to a die

protected by a polymeric coating have not been well researched. Sensors such

as these will play an important role in the next generation of implanted

medical electronic devices and technology for their non-hermetic protection in

a saline submersed environment must be developed. We have examined additioncure polysiloxanes in accelerated temperature-humidity tests and found them to

be premising materials for this application.

INTRODUCTION

Various micro-sensors and stimulation arrays have been developed [1,2, 3, 4, 5]

for use in implanted medical electronic systems. Most of these devices take

advantage of solid-state technology. In particular, photolithography and

integrated circuit technology have been exploited in the design and fabrication

of a number of subminiature structures specifically designed for use in

neuroprostheses. Most of these devices are characterized by thin film

technology and by a variety of metals and substrates not commonly used in the

semi-conductor industry. The electrode arrays developed by Wise et.al. [2]

require that part of the device actually protrude from the electronics/bond pad

area since the electrode elements are distributed along a spear- like shank

(fig. 1). Although considerable effort has been invested in the fabrication of

such devices, less effort has gone into the definition of micro-packaging

technologies which would permit their implanted use. To date, there exists no

established packaging technology for use with implanted micro-prosthetic

devices. The lack of package definition includes electrode array or sensor

coating, micro-cable design and interconnection, interface of the cable with,

and encapsulation of the main package. ; In related applications, the use of

similar "Smart Sensors" has been suggested for use in robotic, automobile,

military, and space assemblies. All of these devices are characterized by subminiature size and the need for all or part of the die to be exposed to the

external environment.

161
Presently, the most widely used and generally accepted method of protection

from body fluids for implanted electronic devices (and for electronic

components in general) is the placement of an impermeable envelope around the

device. The impermeable envelope usually takes the form of a welded hermetic

can and this approach has been quite successful in pacemakers because it

provides complete enclosure of the inplant by the enveloping material. In

spite of these advantages, the welded can is practical only in those devices

which can afford a bulky case. Electrical connections through the wall of the

can are usually provided by glass-on-metal feed-throughs. The inplant's

entering or exiting cables or wires are connected to the feed-throughs and the

exterior, of the metal can is sealed with an encapsulant or coating such as an

epoxy resin or silicone polymer. As the size of the inplant is reduced, and

the number of exiting conductors increased, the number of feed-throughs become

a determining factor in the size of the final package. For example, the feedthrough and associated encapsulation for a pacemaker can account for as much as one-third the total package volume. Unlike a pacemaker, subminiature

structures, such as the thin-film devices described above, depend upon the

integration of electronics and transducer, and their primary performance

advantages such as low noise and unique function are directly related to their

size; the use of metal cans 'and similar conventional integrated circuit

packaging technologies is precluded.

Encapsulants are an alternative to the welded can. A properly chosen

encapsulation system can provide adequate corrosion protection for many types

of electronic assemblies and a number of organic barrier coatings and

encapsulants have been used to protect electronic systems from deleterious

effects of water vapor including products based upon epoxy, imide, and silicone

polymers. The latter, especially those polymerized by condensation mechanisms

have gained wide acceptance for nan-hermetic chips [6] . However, in implants

the failure incidence of encapsulants (especially silicone polymers) for

electronic devices is much greater than in nonmedical devices such as plasticpak integrated circuits, even when the latter are subjected to accelerated

stress tests. It is not clear whether this poor success rate is due to

improper application and processing of the encapsulants or to material

inadequacies. Silicone polymers are often misused in the sense that they are

often inappropriately regarded as a vapor barrier. However for any system, the

high failure rate, undoubtedly, is a result of the severe exposure conditions

that exist in a physiological environment. Hot only is the inplant exposed to

constant submersion in an aqueous system, but the electrolytic environment and

ionic mobility present a milieu conducive to the electrical leakage currents

which result in corrosion. One advantage of the implanted environment over

other applications is the constant 37°C temperature.

Figure 2 illustrates the basic problem. The sensor device of figure 1 is shown

using hermetic and nan-hermetic packaging techniques. The arrows indicate the

obvious locations of leakage of the hermetic package: at the flexible lead-out

and at the exit of the sensor portion of the device. Using a non-hermetic

approach, the die-coat might be made of the same material as the lead-out,

eliminating one interface. The remaining exit can be protected by using a

coating which can provide inactivation of the normally hydrophylic surface

sites by chemical reaction with or maintained adhesion to the protected portion

of the die.

Silicones are presently being investigated at our laboratory as candidate

materials for integrated sensors. Our ongoing work is directed at the

162
identification of critical material parameters for the selection of existing,

as well as the design of new materials. Based upon the theory that moisture

protection by encapsulants is directly related to the adhesive bond between the

encapsulant and the underlying surface, adhesion testing has emerged as the

primary method of encapsulant qualification for implant use. Pacemaker

manufacturers have performed lap-shear and butt-tensile testing of selected

substrate/encapsulant combinations after high humidity exposure and these tests

relate the bond strengths to the corrosion protective ability of the

encapsulant [7]. Donaldson [8] has also investigated the effects parameters

such as electric field strength [9], and tensile forces within actual implants.

Data from tests upon silicones in our laboratory indicate that bond strength

testing is necessary but insufficient to qualify an encapsulant for corrosion

control of electronic devices [10]. Corrosion is an electrochemical process

whose prevention depends upon the ability of the encapsulant to prevent the

flow of electrical leakage currents. Electrical leakage current tests combined

with bond strength tests provide not only a measure of the polymer's ability to

remain in contact with the protected surface, but also act as an insulator to

electrical leakage currents which inevitably will result in corrosion.

Previous applications of silicones for electronic encapsulation have, for the

most part, been restricted to condensation-cure silicones. In this laboratory

our preference has been for the addition-cure silicone materials. They can be

cured very rapidly, in some cases a matter of minutes, and liberate no byproducts making them useful for closed-container molding. In contrast, the

condensation-cure products liberate a cure by-product, usually acetic acid or

alcohol and cannot be cast into thick sections. In addition, acetic acidevolving products have been implicated in the oorrosicn of numerous military

encapsulated electronic assemblies.

METHODS

We evaluated 6 different silioone elastomers in conjunction with 3 surface

primers in accelerated temperature-humidity tests. The materials and their

designations are as follows:

Surface Primers

Designation Name Manufacturer Formulation

Type A Primeooat 1200 Dew Coming Si lane-Silicate Blend

Type B Cherniak 607 Lord Chemical Blended Silanes

Type C SS 4155 General Electric Ethylorthosilicate

Silicone Elastomers

Designation Name ftfenufacturer Cured Hardness

Durcmeter (Shore A)

Type 1 FTV-615 General Electric 45

Type 2 Sylgard 184 Dow Coming 35

Type 3 M3X-4-4210 Dow Coming 25

Type 4 96-083 Dow Coming 63

Type 5 Sylgard 567 Dow Corning 38

Type 6 PTV-3140 Dew Coming 22

*M3X-4-4210 was blended with 10% Sylgard 527, by weight, to decrease

viscosity for casting purposes.

163
The first 5 silicone elastoners are addition-cure products. The 6th material

is a condensation-cure product which has been previously used for medical

implant protection [11]. As verified by lap shear tests, (described below),

the first 3 elastomers provide minimal adhesion in the absence of a surface

primer. The last 3 are self-priming materials. To permit visual examination,

we tried to select clear materials. All of the elastomers are clear except for

types 5 and 6. Types 1, 2, and 3 were selected for test with each of the

primers, types A, B, and C. In addition, type 4 elastomer was used as a primer

for types 1, 2, and 3. All elastomers types 1-6 were also tested without

primers.

Testing was performed by encapsulation of 4 PWB, 6-10, epoxy-glass

interdigitated test pattern substrates per test group. The patterns selected

were taken from the IPC B-25 standard test board, patterns A and B. These

patterns use interdigitated line widths and spacings of 0.006" and 0.012"

respectively. Two single-sided A patterns (0.006" lines and spacings) and two

single-sided B patterns (0.012" lines and spacings) were prepared for each

elastomer-primer combination tested. Following encapsulation, each of the

patterns was continuously submersed in 85° C water and placed under a continual

voltage stress of 20VDC. Electrical leakage current (under 9VDC bias) was

measured for each of the test substrates on a daily basis.

Preparation of each test substrate was performed as follows: A 10" long,

0.005" diameter Kynar (polyvinylidene fluoride) insulated wire was soldered to

each electrical half of the substrate. The substrate was then completely gritblasted (40 PSI) using 240 mesh (50um) aluminum oxide grit. Following gritblasting, the substrate was cleaned in a series of solvents as follows:

Methylene Chloride-1 min. wash. Isopropyl Alcohol rinse, Acetone-1 min. wash,

Isopropyl Alcohol rinse. Freon TMS-2 min. ultrasonic clean in a Branson B-220

ultrasonic cleaner. Freon TF running rinse. During the cleaning procedure the

substrate was handled, with gloves, only by the ends of the connecting wires.

After cleaning, a vacuum-bakeout was performed for 12hrs at 120°C. Upon

removal from the vacuum oven, designated substrates were primed with the

appropriate primer according to the manufacturer's instructions. Each of the

manufacturers was contacted directly in order to verify the procedure as

outlined in the data sheet. A substrate ready for encapsulation was placed in

a half-open mold which supported the substrate horizontally by its wires. The

appropriate encapsulating elastomer was mixed and de-aired and poured into the

mold under vacuum casting. This procedure produced encapsulated substrates

with 0.100"+0.015" encapsulant thickness over and under the substrate. The

diameter of the mold was such that a thickness of at least 0.3" of encapsulant

covered the substrate's edge. As a result of the vacuum casting no bubbles

were visible in the encapsulated substrates. Curing of the addition-cure

elastomers was by forced-air convection at a prescribed temperature of 120°-

165°C depending upon the particular elastomer cured. Curing of the type 6

material was done in a humidity chamber of >90%rh for 72hrs. The specific

combinations of primers and elastomers tested are listed in table 1. Type 4

elastomer was used as a primer due its previously demonstrated high bond

strength in experiments in our laboratory. It was applied in a very thin

(<0.005") coat over the substrate and cured before elastomer casting.

A Keithley 61QA electrometer with a shielded box and a 9VDC battery comprised

the leakage current measurement apparatus. Resolution of the system was at

least O.lpA, however measurements less than 0.5pA were not recorded. Due to

164
the low levels of measured leakage current, a shielded box was essential to the

measurement system. The volume resistivity of most silicones shows a strong

temperature dependence. Therefore all leakage current measurements were made

at room temperature. The substrates were removed from the water bath for a

period of lhr prior to measurement. Visual examination was done on a periodic

basis, documenting visual evidence of discoloration, debonding, or corrosion.

A dry lap shear bond test, as described in ASTM-D905 (modified) , to G-10 for

each of the tested materials was performed on an Instron Universal Testing

Instrument. Double lap shear samples were prepared in the same manner as the

test substrates.

RESULTS

The measured values of leakage current for each of the 72 test substrates over

the initial 2 month test period are listed in table 1. In addition, visual

test results are reported at days 30, 45 and 60. For each group of 4 samples,

samples 1 and 2 are the A pattern (0.006" spacing), and samples 3 and 4 are the

B pattern (0.012" spacing). The initial baseline leakage currents for each of

the samples was in the low pA region, and for many of the samples, the

measurement error below 0.5pA was probably greater than the actual leakage

current. The Kynar insulated wire for connection to the substrates was a poor

choice. Submicroscopic defects in the insulation resulted in discoloration and

corrosion in the wires of a number of the samples. Seme of the samples failed

due to wire breakage. However, subsequent testing on specially prepared

samples indicated that the measured values of leakage current were not affected

by the wire discoloration; these samples continued to give consistent readings

up to the day of wire breakage.

All samples primed with type B primer showed visible corrosion at a very early

date. Within one week of water submersion, all of these samples showed some

signs of deterioration. In each case, the side of the pattern maintained at

positive potential was the first to discolor and then corrode. In contrast

none of the samples primed with type 4 material showed visible deterioration at

day 60. With the exception of the type B samples, all of the other samples

failed to show any noticeable deterioration until one month into the test. At

this time, most of the types A, B, and C primed samples showed discoloration

and debonding. Correspondingly their leakage currents, at day 30, were an

average of 35 times greater than those samples primed and encapsulated with the

self-priming elastomer types 4, 5, and 6. The samples on which no primer was used, and the encapsulating material was

nonpriming are perhaps the most interesting. Their leakage currents were

consistently lower than the types A, B, and C primed samples in spite of the

fact that adhesion as tested in lap-shear tests was considerably less than the

primed samples. Results frem the lap shear bond tests are presented in table

2. Note that when used without surface primers, elastomer types 1, 2, and 3

demonstrated the lowest relative bond strengths when compared to the primed and

self-priming samples. However, the leakage currents from the unprimed samples

were an average of 50 times lower (day 45) than those primed with primer types

A, B, and C, and their overall visual appearance was superior to that of the

primed samples. The best performance was seen in types 4, 5, and 6 samples.

None of these samples showed any signs' of significant deterioration-visual or

electrical, at day 60. Type 6 material consistently showed overall superior

performance.

165
PRIMER ENCAPSULANT SAMPLE# ELECTRICAL LEAKAGE IN AMPERES FOR ELAPSED TIME

0 6 hrs day .15 day 30 day 45 day 60

TYPE A TYPE 1 1

2

34

<0.5p

3. Op

3. Op

2. Op

640n

140n

150n

74n

700n

360n

120n

lOOn

200n d

74n d,db

66n d,db

82n

42n d,db

42n d,db

12n d,db

9.4n

41n d,db

41n d,db

9.8n d,db

9.4n

TYPE 2 1 <0.5p 20n 400n 260n d 62n d 39n d,db

2 <0.5p 20n 540n 400n d 83n d 59n d

3 <0.5p 5.8n 160n 86n c,db 24n c,db 18n c,db

4 l.Oo 20n 220n 82n 22n 22n db

TYPE 3 1 l.Op 320n 3.2u 620n d 86n d,db 90n c,db

2 64p 1.8u 2. On 440n d 26n d 40n d

3 3. Op 32n 620n 300n 86n 80n

4 1.5p 120n 320n 510n 43n db 36n db

TYPE B TYPE 1 1 2.9p 160n 8.8n 74n c,db 3. On c,db 1.6n c,db

2 l.Op 96n 32n 68n c,db 9.4n c,db 1.7n c,db

(note 1) 3 1.5p 6.4n 2.8n 2.2n c,db 960p c,db 290p c,db

4 2. Op 700n failed 0 dav 10, current >100uA

TYPE 2 1 <0.5p 7.4n 16n 9.4n c,db 1.8n c,db 620p c,db

2 l.Op 7.2n 8. On 8. On c,db 2.8n c,db 1.8n c,db

3 <0.5p 2.6n 2. On 3.8n c,db 440p c,db 420p c,db

4 2.5d 540p 3. On 3.4n c.db 16n c.db 18n c.db

TYPE 3 1 <0.5p 3.5n 5.4n 2.8n c,db 2.5n c,db 2.5n c,db

2 <0.5p 140n 14n 4. On c,db 540p c,db 420p c,db

3 1.5p 4. On 2. On 420p c,db 150p c,db 180p c,db

4 <0.5p 5.4n 2.2n 3. On c.db 280p c.db 210p c.db

TYPE C TYPE 1 1 7. Op 38n 140n wire failure

2 5. Op 32n 220n 40n d 34n d 30n d

3 <0.5p 14n 68n 24n d 8.8n d 8.6n d

4 2.0d 7.8n 48n 28n d 7.6n d 6.2n d

TYPE 2 1 <0.5p 28n 320n 44n 370n wire failure

2 <0.5p 540n 2.2u 550n c,db 360n c,db 200n c,db

3 <0.5p 8.4n 120n 68n db 14n db 14n d,db

4 2.5d 50n 220n 52n db 26n db 22n db

TYPE 3 1 65p 1.2u 660n 140n db 18n db 22n db

2 440p 190n 1.4u 160n d 180n d 160n d

3 HOp 840n 140n 24n 18n 16n

4 40p 600n 900n 72n 160n c.db 140n c.db

TYPE 4 TYPE 1 1 <0.5p 50p 5.4n 3.4n 720p 520p

2 <0.5p 60p 8.2n 1.4n 950p 280p

3 <0.5p 4. Op 16p 50p wire failure

4 l.Op 150p 120d 14n 120d 58p

TYPE 2 1 <0.5p 80p 12n 6. On 3.6n 560p

2 <0.5p 20p 36p 8.8p 54p 60p

3 <0.5p 220p lOn 440p 620p 2.9n

4 <0 .5p 64p 30p 5.4n 26p 38d

TYPE 3 1 <0.5p 14p 260p 120p 39p 35p

2 5. Op 46p 1.2n 1.6n 220p 200p

3 8.5p 7. Op 32p 30p 22p 32p

4 <0.5p 14d 30p 68p 18p 26d

NONE TYPE 1 1 <0.5p 50p 5.6n lOOn db 500p db 360p db

2 <0.5p 220p 32n 5.4n db 2.9n db 2.7n db

3 <0.5p 3. Op 40p 320p db 28p db 72p db

4 l.Oo 40d 32n 5. On c.db 2.2n c.db 2.8n c.db

TYPE 2 1 l.Op 1.2n 32n 400n db 2. In db 7.2n db

2 1.5p 1.2n 8.8n 84n db 320p db 600p db

3 1.2p 4.2p 56p 66p 21p 47p

4 <0.5p 3.0o 30p 30p 22p 19p

TYPE 3 1 5. Op 8. Op 5. On 24n 720p 840p

2 l.Op 12 P 300p 660p 380p 170p

3 3.5p 14p 60p 40p 30p note 2 1.2u

4 1.5p 4.5p 22d 60p 56 p 50p

TYPE 4 1 <0.5p 1.4u‘ 7.2n 1.8n 710p note 2 4. On

2 <0.5p 56p 84n 12n 12n wire failure

3 <0.5p 3. On lOn 840p 180p 200p

4 <0.5p 1.8p 22p 25p 20p 23p

TYPE 5 1 <0.5p 160p 3. On 500p na 420p na 340p na

2 <0.5p 52p 320p 500p na 600p na 580p na

3 <0.5p 56p 2.2n 180p na 35p na 76p na

4 <0.5p 14p 48p 200p na 45p na 46p na TYPE 6 1 <0.5p 22p 50p 40p na 35p na 38p na

2 <0.5p 60p 120p 7 Op na 15p na 32p na

3 <0.5p 6 .Op 16p 40p na 18p na 38p na

4 <0.5p 6. Op 2fip_ 46 p na 23p na 28p na TABLE 1 Electrical leakage current measurements for interdigitated G-10 silicone

encapsulated substrates over a 60 day period. Visual examinations made at day 30,

45 and 60 are coded as follows: d=discoloration of the conductors, db=debonding of

the encapsulant, c=corrosion, na=not applicable. Note 1: All B-primed substrates

showed corrosion at day 10. Note 2: Foil-pattern defect caused local metal bridge.

166
PRIMER ENCAPSULANT LAP SHEAR (PSI) PRIMER ENCAPSULANT LAP SHEAR

TYPE A TYPE 1 451 TYPE C TYPE 2 339

TYPE 2 389 TYPE 3 163

TYPE 3 149

NONE TYPE 1 41

TYPE B TYPE 1 512 TYPE 2 46

TYPE 2 276 TYPE 3 80

TYPE 3 111 TYPE 4 575

TYPE 5 201

TYPE C TYPE 1 292 TYPE 6 291

Table 2: Lap shear test results for G-10 substrates and various primerelasterner combinations . (ASTM-D905 modified)

DISCUSSION

The rapid initial rise of leakage to steady-state values characteristic of most

of the test samples was not surprising in light of the relatively high water

vapor permeability of the silicones. Within 6hrs of water submersion most of

the samples demonstrated currents of about the same order of magnitude as those

which persisted throughout the test.

The lack of performance of all of the primed samples and the corresponding

superior performance of the unprimed samples using types 1, 2, and 3 elastomers

raises serious questions concerning the validity of adhesion testing in the

selection of silicones for corrosion control of electronic assemblies. The

poor performance of the primers A, B, and C, is perhaps easier to explain than

the good performance of the unprimed samples. The manufacturers' original

purpose of these primers was for bonding of silicones in non-electrical

applications. The manufacturers presently provide no data concerning their

electrical properties. One might postulate that these primers contain watersoluble contaminants, and that although they may remain bonded, their bulk and

surface resistivities drop by orders of magnitude in the presence of moisture.

There would seem to be no other reasonable explanation as to how groups such as

type A-type 3 could sustain high levels of leakage current without early

significant visible deterioration. Condensation of water at a debonded

polymer-substrate interface sufficient to produce leakage currents in the near

uA region would have to result in rapid conductor corrosion. Although we have

not, as of yet, performed tests to determine the degree of possible contamination for each primer, the results of this study and others seem to support the

concept that the primers degrade the performance of the elastomers from a

corrosion control, and an electrical insulation standpoint. The use of primers

such as type C in military high-voltage assemblies has been associated with

drift of high resistance circuits such as high-voltage voltage dividers [12].

The degraded performance of materials such as types 1, 2, and 3 with the use of

a primer cannot be more clearly seen that with type B primer. However, in

spite of the rapid deterioration of the type B primed samples, they did not

have the highest leakage currents. Although low electrical leakage is a

necessary requirement for prolonged substrate survival, a weakness of leakage

current measurements as a sufficient criteria of encapsulant acceptance is the

difficulty in determining the current density across the substrate. Local

sites of high current flow can result in local corrosion without causing

correspondingly high values of total leakage. Low total current as well as a

uniform current density is necessary for acceptance of an encapsulant. Note

167
that even for those samples which did not demonstrate early visible signs of

corrosion, but did show elevated levels of leakage currents, the protecting

primer-elastomer combination may not be acceptable for many high-impedance

assemblies. An upper level of 60nA was chosen in this study as an acceptable

total leakage current level. This represents a 5% error in the current which

might flow in a lOMohm resistor with a voltage differential of 12VDC. None of

the primers A, B, or C could meet this limit, whereas the sanples with types 4,

5, and 6 materials consistently met or exceeded this limit.

Perhaps the most surprising sanples were the unprimed substrates encapsulated

with types 1, 2, or 3 elastomers. The low bond strengths as seen in table 2

for these materials did not seem to be significant in their corrosionprotective ability. It may be that low substrate surface tension, with a

correspondingly high degree of wetting by the protecting encapsulant is more

important than the actual interfacial bond strength. Maintaining this surface

wetting is obviously enhanced by strong adhesion, especially in assemblies with

complex geometries and high local mechanical stresses. However, in certain

assemblies adhesion is often lost due to the presence of sharp comers and

close component spacings, characteristic of high-density FWBs. It may be that

surface wetting could also be maintained by using a low durometer elastomer

(not necessarily self-priming) as a "priming layer" with an overcoat of a more

rigid elastomer. It is interesting to note that the materials which performed

the best in this study were not only the self-priming ones, but also the ones

characterized by low durometer readings. Both types 5 and 6 materials are

self-priming, and of relatively low durometer (38 and 22) as compared with type

4 having a durometer of 63. Type 5 and 6 materials performed better than type

4, with type 6 (of lowest durometer) currents lower by approximately an order

of magnitude. Type 6, however is a condensation-cure material and cannot be

completely cured in closed molds or in thick sections. Most of our implant

devices are cast, and addition-cure materials are preferred.

Rigid cleaning protocols are essential to low surface tension. Cleaning of the

encapsulated surface prior to encapsulation may be the single most important

process step in the use of any encapsulant. Contaminants present on the

surface including non-polar organics (oils, grease), polar inorganics (water

soluble salts, chlorides, etc), and insoluble particulate matter (dirt, dust),

will not only interfere with the bonding of the encapsulant to the surface but

are themselves sources of ionics which when put into solution will result in

rapid deterioration. The presence of water soluble contaminants upon the

encapsulated surface provides the ideal conditions for the initiation of

corrosion. This is especially true when using the semi-permeable silicones.

As a demonstration of this, we encapsulated a substrate (B pattern) using type

1 elastomer without any cleaning performed. Solder fluxes, finger prints, and

other contaminants normally present during assembly were not cleaned off. This

sample showed visible corrosion within 12hrs of water submersion. In contrast,

the cleaned unprimed type 1 samples in this study survived over 60 days of

exposure with comparatively little degradation.

The degree of life acceleration that this test represents is uncertain.

Fitting data to a temperature dependent first-order exponential (Arrhenius)

failure curve is often inaccurate due to initial model assumptions. Depending

upon the estimate of activation energies used, and the criteria used to define

failure, an acceleration factor in a range of 17-50 may result [13] . Based

upon a frequently sited rule of thumb, that reaction rates double for every

10°C, an acceleration factor of 32 would be used. We have no evidence at this

168
time that a first-order exponential model is valid, nor that the 85°C

accelerated test does not, in itself, induce stresses which would normally not

be possible during the service life of the device. This is a criticism which

is often sited against accelerated life tests. We do, however, have

preliminary data from other humidity studies in our laboratory in which the

time to corrosion for the type B primed substrates at 65°C was measured. Based

upon these studies an acceleration factor of 25 was calculated for the 85°C

test.

CONCLUSIONS

As part of our continuing program of evaluation of appropriate encapsulants for

medical implants as well as non-medical electronic high-density assemblies, we

shall pursue the question of surface wetting vs. strong adhesion as a criteria

for selecting encapsulants suitable for corrosion control. We are investigating

the use of silicone gels in place of surface primers in applications where a

low^-duroneter material, with high wetting properties, may provide good humidity

protection in the presence of high local mechanical stresses.

Low leakage current for the qualification of an encapsulant as a corrosion

resistant coating is a necessary, but not sufficient, condition. The use of

adhesion tests as a sole qualification of an encapsulant, used essentially as an insulator, is at minimum incomplete, and sometimes invalid. The bond

strengths seen for types A, B, and C primer with their poor corrosion resistant

performance demonstrate the inappropriate nature of classical bond strength

tests. High bond strength accompanied by low leakage current is probably an

indication of a high degree of surface wetting, good cleaning protocols, and

low encapsulant contamination. All of these factors seem necessary for good

corrosion control using silicone encapsulants

.

REFERENCES

[1] K.D. Wise, "Multichannel multiplexed intracortica1 recording arrays,"

9th Quarterly Report, NIH-NINCDS-N01-NS-1-2384, 1984.

[2] K.D. Wise, J.B. Angell, and A. Starr, "An integrated-circuit approach to

extracellular microelectrodes, " IEEE Trans. Bicmed. Eng., EME-17,

pp . 238-246, July 1970

.

[3] M. Esashi, and T. Matsuo, "Integrated micro multi-ion sensor using field

effect of semiconductor, " mnn Trans. Bicmed. Eng., EME-25, pp . 184-191,

ftferch 1978

.

[4] S.D. Moss, C.C. Johnson, and J. Janata, "Hydrogen, calcium, and potassium

ion-sensitive FET transducers : A preliminary report,

Trans. Bicmed. Eng., EME-25, pp. 49-54, January 1978.

[5] H.D. Mercer, and R.L. White, "Photolithographic fabrication and

physiological performance of microelectrode arrays for neural

stimulation," JKnri Trans. Bicmed. Eng., EME-25, pp. 494-500, November 1978.

[6] D.S. Peck, and C.H. Zierdt, "The reliability of semiconductor devices in

the Bell System," Proc. IEEE, 62, 2, pp. 185-211, Feb. 1974.

[7] K.E. Cobian, et. al. "Epoxy adhesives for implantable medical devices,"

Adhesives Age, pp. 17-20, December 1984.

[8] P.E.K Donaldson, "The underwater life of joints between seme adherend and

adhesive materials in neurological prosthesis-making, " Proc. Int

.

Conf . Bicmed. Polymers, pp. 143-150, July 1982.

169
[9]

P.E.K. Donaldson, and E. Sayer, "Silicone rubber adhesives as encapsulants

for microelectronic implants," Med. Biol. Eng. Comput., 15, pp. 712-715,

1977.

[10] P.R. Troyk, M.J. Watson, and J.J. Poyezdala, "Humidity testing of silicone

polymers for the corrosion control of implanted medical electronic

prostheses," Proc. ACS, Div. Polymeric Mat. Sci. & Eng., vol.53,

pp.457-464, Chicago, Sept. 1985.

[11] P.E.K. Donaldson, and E. Sayer, "A technology for implantable hermetic

packages. Part 1: Design and materials," Med. Biol. Eng. Ccmput., 19,

pp. 398-402, 1981.

[12] Personal canmunication with R. Swendsen, Northrop Corp. D.S.D., Rolling

Meadows, IL.

[13] P.E.K. Donaldson, and E. Sayer, "A technology for implantable hermetic

packages. Part 2: An implementation," Med. Biol. Eng. Carput., 19,

pp.403-405, 1981.

CHARACTERIZATION OF IMPLANTED ENVIRONMENT

* AQUEOUS IONIC "SOUP" (Na, Cl, K, Macrophages)

* CONSTANT SUBMERSION

* CONSTANT TEMPERATURE: 37 DEGREES C.

* MINIMAL DIFFERENTIAL MECHANICAL STRESS

MATERIAL REQUIREMENTS FOR IMPLANTED ELECTRONICS

* PROVIDE NON-HERMETIC PROTECTION OF THICK AND THIN

FILM DEVICES WITHOUT SIGNIFICANT VOLUME INCREASE.

* ABLE TO WITHSTAND "SALTWATER" ENVIRONMENT

CONTINUOUSLY.

* PROCESSING ADAPTABLE TO FLEXIBLE LEAD-OUTS

WITH MINIMAL BULK.

* BIOCOMPATIBLE, OR ABLE TO BE COATED.

170
Bond Pads

100 u

Figure 1 . Silicon Probe for neural sensing as designed by Wise et. al. [2]. The bond pad

area must be protected, however the electrode pads must be exposed to a

saline environment.

HERMETIC CONFORMAL

Figure 2. The sensor of figure 1 with hermetic and non-hermetic packaging. The Arrows

indicate the probable sites of leakage for the hermetic package.

171
ELECTRONIC IMPLANT MOISTURE-PROOFING VARIATIONS

VARNISH

ACRYLIC WITH PVC COATING

ACRYLIC WITH EPOXY COATING

PMMA WITH BEESWAX

EPOXY

SILICONE RUBBER WITH WAX

GLASS WITH EPOXY COATING

PARYLENE C (GLOW DISCHARGE PRIMER)

POLYIMIDE

SILICONES

WELDED CAN

BEESWAX

TEFLON

IMPORTANT MATERIAL PARAMETERS FOR SILICONES

* ADHESION TO PROTECTED SURFACE

* LOW TO MODERATE SHORE A DUROMETER

PREFERABLY ADJUSTABLE

* SMALL NUMBER OF LOW-ORDER CHAINS

* CLEAR OR TRANSPARENT

* CONTROL OVER PRIMING

172
4.1 PIN SELECTION CRITERIA FOR A.C. CONDUCTANCE TECHNIQUE

N.K. ANNAMALAI

RADC/ESR, Hanscom AFB, MA 01731

EUGENE BLACKBURNE

RADC/RBRE, Griffiss AFB, NY 13441

S.M.R. ISLAM

Clarkson University, Potsdam, NY 13676

ABSTRACT

An a.c. conductance technique used to determine the moisture in

CERDIP packages is described. A circuit model is used to explain the observed

experimental behavior. The conductance method measures the conductance

between two pin pairs which have long parallel adjacent metallization runs. A description of the criteria for pin pair selection is given. The conductance

method can be applied by selecting more than one set of pin pairs and connecting them in series or in parallel. The sensitivity of measurement is improved

by using a series connection, and this is seen by employing a figure of merit

for each connection.

INTRODUCTION

Moisture in hermetically sealed IC packages and hybrid packages is a known realibility hazard. An a.c. conductance method using a lock-in-amplifier

(LIA) is superior to all the other available methods. The advantages of this

method over other methods have been described in earlier works [1, 2]. The

present conductance technique measures the conductance between any two pins

of the chip which do not have any electrical component between them and which

have long parallel adjacent metallization runs. An LIA is used to measure

the conductance at 100 Hz and at 100 mV. The conductance is monitored as a function of temperature when the chip is taken through a cooling cycle followed

by a heating pycle [1, 2].

Pin pair selection is important for the effective implementation of this

technique. Further, the sensitivity (to measure low levels of moisture as low

as 500 ppmv) can be increased by connecting a number of pin pairs in series.

MEASUREMENT PRINCIPLES

Conductance and capacitance of a device can be measured simultaneously

by using an LIA. A voltage from a function generator is fed to the currentsensitive preamplifier and the device under test (DUT) connected in series.

The voltage at the output terminals of the preamplifier is proportional to

the current through the device. The magnitude and phase angle of the voltage

are measured by using an LIA. Comparing this voltage with the reference

applied signal, the LIA provides both the , magnitude and phase angle. The

LIA locks itself on the reference frequency provided by a function generator.

173
Two meters on the face of the LIA read either the magnitude and phase,

or in-phase and quadrature phase voltages in equivalent d.c. form. The

in-phase voltage is directly proportional to conductance, and the quadrature

voltage is directly proportional to capacitance.

An input protective diode on the chip or a diode placed close to the

package can be used for temperature measurement. A constant current is

pumped through the diode, and the voltage drop across the diode is then given

by a relationship of the form Vq = AT+B where A and B are constants and T is

the temperature in K. The calibration method used is descried in the latter

part of this section.

Simultaneous measurement of temperaXure and conductance is not possible

due to the common return path for both the signals. Hence time multiplexing

of the temperature and conductance is essential for successful implementation

of the conductance technique. To time-multiplex, a dummy device and the device

to be tested were used to avoid meter transients during on/off cycles. In one time slot, when the temperature of the test device was measured, conductance

of the dummy was measured, and at the next time slot measurements of the

devices were reversed.

When a rapid cooling of the device is desired, the time-multiplexing

procedure is not possible with the data acquisition unit we presently have.

A rapid rate of cooling of the die was found to be essential to improve the

sensitivity of the measurement. In such situations a thermocouple was attached

to the bottom side of the package with thermal cement and used to monitor the

temperature of the die. Due to the temperature gradient between the die and

the bottom of the package, a calibration run for each type of package was performed. An additional calibration experiment was required to calibrate

the diode to measure the temperature. The chamber was kept at a specific

temperature, and the die was placed in the chamber and allowed to equilibrate

with the chamber for a sufficiently long time. A voltage drop corresponding

to the chamber temperature was noted when a constant current was pumped

through the protective diode. The chamber temperture was read by a digital

thermometer. The temperature of the chamber was varied and the corresponding

diode voltage was obtained. From this set of readings, diode constants A and

B were obtained. The die temperature was read by measuring the diode voltage

Vj) =AT+B when the current pumped through the diode was the same as in the

calibration experiment. When a package was cooled rapidly, there was a temperature gradient between the die and the package. For each type of

package, keeping the experimental conditions the same as that of the

conductance method, the temperature at the bottom of the package was obtained

by a thermocouple and the die temperature by the diode voltage. In the

actual conductance technique, temperature was monitored at the bottom of the

package by a thermocouple and the die temperature was thereby determined

from the calibration experiment.

THEORY

The conductance is measured between two selected pins having a large

impedance (>_ 1000 M\l) connected to metal tracks. The metallization

tracks lie on top of silicon dioxide and connect silicon devices. A

glassivation layer on top covers the tracks. These lines are separated from

each other by silicon dioxide and through reverse biased p-n junctions on the

174
The LIA measures the conductance and capacitance between the two pins. G3 includes the pin-to-pin leakage through the ceramic package and the leakage

of all the connecting leads up to the LIA, and this conductance is less

temperature sensitive and very small in practice. G 4 is the leakage between

metallization tracks through Si 02 , and this is also very small. G]^ is the

junction leakage conductance, and its value varies exponentially with the

temperature. In the temperature range we are interested in (0 to -40°C), G]^ is large compared to G 3 and G 4

. The capacitance is due to lead capacitance,

plus the ceramic dielectric capacitance, and a subs trate-to-metal track

capacitance, and a depletion layer capacitance. All the capacitances are lumped together because they add in parallel. When moisture is present in

the package, a conductance G2 is added in parallel to G^ due to dropwise

condensation of moisture on cooling the die. These droplets also add a capacitance C 2 to the substrate, which is shown in parallel with G 2

. The

moisture-induced conductance and capacitance are connected through a switch S,

closed only when moisture is present in the liquid form. The temperature at

which the closing of switch occurs is also a function of the amount of moisture

in the package. The switch closure occurs at a temperature T when water

starts condensing on the die, and this temperature is taken as the dew point

for our purpose. In a dry package condensation never occurs and hence the

switch S is never closed. Further, even though the circuit components are shown as lumped quantities, they are really a distributed parameter system.

Conductance G 2 and capacitance C 2 are there only when moisture is in liquid

form. When moisture is in the solid or gaseous phase these conductance and

capacitance components vanish and hence the switch is not closed under these

conditions. The circuit is helpful in understanding the shape of the

conductance characteristics as a function of temperature. The moisture-induced

conductance G4 should be at least 10% of the G^ for the required sensitivity.

Hence to improve sensitivity, a number of pins pairs can be selected and

connected in series. The capacitance variation was not as sensitive as the

conductance variation and hence this technique is termed a.c. conductance

technique. Even though conductance and capacitance were measured simultaneously,

the discussion pertains to conductance only.

INSTRUMENTATION

An LIA circuit setup was used for moisture measurement. For efficient

data collection, control and recording of the results, a data acquisition

setup was used. An HP 85 microcomputer, and HP 7475A plotter, and HP8290B

printer, and an HP 3421A data acquisition unit with a multiplexer were used

for data collection and control. The data acquisition was completely under

computer control. The experimental data collected were displayed on the screen and a hard copy was obtained from the printer. Plots of* conductance

or capacitance and diode voltage as a function of time were made when desired

froiti the computer memory after the completion of the experiment, with the

help of the plotter. The data acquisition system HP3421A with a multiplexer

permitted us to record the diode voltage (proportional to die temperature)

and the conductance of the device under test (DUT) from the LIA, and to

control the cooling and heating rate of the chip. Due to the common return

path problems encountered in making temperature measurements using a protective

diode on the die and conductance measurements using LIA simultaneously, the

following scheme was used. Along with the device under test, a dummy device

was selected. When temperature was measured on one, conductance was mesured

on the other and vice versa. These measurements were computer controlled

175
silicon substrate. A leakage current of the order of nanoamperes was measured

in the devices we tested. This current is due to leakage through the reverse

biased junctions in the silicon substrate and the isolation islands. This

conductance, when measured as a function of temperature, shows a positive

slope in the absence of moisture. When the chip is cooled, a dropwise

condensation of the water is seen by videotaping the process. These droplets

on the top of the glassivation along with the condensed water in the capillaries

provide an additional leakage conductance path. Therefore the conductance

slope with the respect to temperature is changed from positive to negative when

moisture is present. This sharp conductance change is marked as I in Figure

1. In this figure, the conductance variation is shown (by a thick solid

line) as a function of time. Temperature of the chip, measured by a diode,

is shown (by a dashed line) as a function of time. Water can be in a liquid

state even well below 0°C and the temperature at which this transition occurs

is a function of the amount of moisture in IC packages. The transition of

water into ice is shown as a sharp drop in conductance (marked as II in

Figure 1). If the device is taken through a heating cycle following thd

above cooling cycle, ice melts into water around 0°C. This results in a large increase of conductance (marked as III in Figure 1). Further heating

results in water transforming into the gaseous state, and this is exhibited

by a sharp drop in conductance value (marked as IV in Figure 1). The four

sharp transitions in conductance value when the device is taken through a cooling cycle followed by a heating cycle are seen when moisture is present

in the IC packages. If there is no moisture in the package then the conductance

follows the short and long dashed lines in the conductance plot (also marked

as "NO WATER PRESENT"). This characteristic does not show any sharp transition.

Since the conductance of a dry package is due to junction leakage, it is given

by

G °c exp (-Eg/kT)

where G = the conductance

Eg = the band gap energy

k = Boltzmann's constant

f = Temperature in °K

The conductance of a dry package therefore will decrease exponentially as temperature is decreased. This has been verified experimentally.

MODEL

A physical model is shown in Figure 2. The metallization tracks lie on

Si 02 at certain sections and make contact with the silicon devices at certain

sections. These are shown in Figure 2. As the die is cooled, water condenses

into the capillaries of the glassivation layer and condenses as droplets on

top of the glassivation layer. An equivalent circuit was developed to explain

the conductance characteristics seen in Figure 1 along with the physical

model shown in Figure 2. The equivalent circuit is illustrated in Figure 3.

176
through multiplexer boxes and software programs. Further, software programs

were developed to take data from the DUT only and the dummy device was used

to avoid measurement transients. Data acquired were also stored on cassette

tapes

.

PIN SELECTION

Even though theoretically any pair of pins not electrically connected

through internal components and having long parallel adjacent metallization

runs is suitable, a scheme to select the best pin pairs is presented. Further

the mode of connections to be used for increased sensitivity is also discussed

The pin selection criterion is discussed using a chip layout diagram

shown in Figure 4. The layout is for a three-input three-NAND gate TTL

circuit. For the first NAND gate, 3, 4 and 5 are input leads and 6 is the

output lead. The three input leads for the second NAND gate are 9, 10 and 11

and the output lead for it is 8. The third NAND gate input leads are 13, 1 and 2 and the output lead is 12. The pin numbered 7 is the ground lead and

14 is the lead for power supply Vcc.

For effective implementation of the a.c. conductance technique, select

pin pairs (a) having lowest conductance value and (b) having a close and a long parallel metallization tracks. This is the pin-pair selection criterion.

As a first step, conductance values between various pin-pairs were measured.

Conductance values between all the possible pin pairs combinations of this

CERDIP package are measured. Only a few measured values are shown in Table I.

Let us consider pin pair combinations 1-2, 1-3,.... 1-14. The ground pin

7 has the longest metallization line (refer to Figure 4), but its leakage

conductance is also very high with respect to any pin and hence it is rejected

In this case the conductance between pins 1 & 7 is 27.9 x 10“^S. Pin pairs

1-3, 1-4, 1-5, 1-6, 1-8, 1-9, 1-10, 1-11, 1-12 are not suitable because there

are no metallization tracks close by between these pin pairs. Pin 1-14 is

dropped due to large leakage conductance between the pins. Pin pairs 1-2 and

1-13 are selected because they have a low leakage conductance and have a nearby metallization track (see Figure 4). Similarly, pin pair 2-13 is

selected because of low measured leakage conductance and nearby metal tracks.

By similar reasoning pin-pairs 3-4, 3-5, 4-5, 9-10, 9-11, and 10-11 are selected. All suitable pin pairs selected using the pin pair selection

criterion for moisture measurement are highlighted in the table. The metal

tracks need to be close by and should be long and parallel to register a high

value of moisture-induced conductance. The pin pairs 3-4, 3-5 and 4-5 belong

to gate I, pin pairs 9-10, 9-11 and 10-11 to gate II and 1-2, 1-13 and 2-13

to gate III. In the chip layout gate III is on the left lower end. The

gates I & II are on the right side of the chip. The water condenses on the

chip when it is cooled, but where it condenses on the chip is dependent on

the way the chip is mounted in the package. If the chip is mounted slightly

slanted to the package bottom, the surface closer to the bottom of the package

will condense more water droplets. In this case, let us assume gate III is

closer to the bottom of the package, and we are using pin pairs corresponding

to gates I or II. The sensitivity will be poor and will vary from one package

to the other, because chip mounting is not identical for every package even

though we are considering exactly identical chips housed in each package. In

other words, selecting the same set of pins will not provide the same sensitivity for all packages containing the same type of chips. Hence in

177
order to improve the sensitivity of measurement and to reduce the effect of

chip mount variations a number of pin pairs are selected and are connected in

series or parallel combination. These connection modes and their advantages

measured by a figure of merit are discussed in the next section.

CONNECTION MODES

Pin pairs 1-^2, 1-13, 2-13, 3-4, 3-5, 4-5, 9-10, 9-11 and 10-11 were selected as suitable pin pairs for moisture measurement using the pin pair

selection criterion laid out in the previous section. Of these pin pairs,

input pins belonging to each gate are shown in the layout diagram (Figure 4).

Since the gates are presumably isolated from each other, pin-pairs from one gate can be connected to the pin pairs of another gate, in series or in

parallel. The results of such a study are illustrated in Table II. In

experiment 1 (marked as NO. 1 in the Table II), two pin pairs 1-2 and 9-10

were selected and were connected in series. Actual connections are given

under mode of connection. In this case, it is shown as 2-10 < l-9> under the

mode of connection, and it implies that 1 and 9 are shorted and the conductance

is measured between 2 and 10. The conductance of the above series connection

and the diode voltage as a function of time are shown in Figure 5. The

measured conductance is the wet conductance (Gy) and is shown by a solid

line. When there is no moisture inside the package, the conductance will

decrease continuously as the temperature of the chip is dropped, as shown by

the solid line, then by a dashed line followed by a solid line (Figure 5).

The dry conductance Gd shown in Table II is the value of the measured conductance just before the onset of condensation of moisture when the chip is

being cooled. The difference between the wet conductance and dry conductance

is AG and the figure of merit (M) is AG/Gd

. The figure of merit (M) is an important parameter to characterize the senstivity of a particular connection.

For this particular series connection G

d was measured to be 1.92 x 10

^

S,

and AG was measured to be 0.37 x 10“-*-^ S and the figure of merit (M) was determined to be 0.40. In experiment 2, only one set of pin pairs 1-2 was selected. The figure of merit for this connection was the lowest (M = 0.07).

A parallel connection was adopted in experiment 3, pin pairs selected were 1-

2 and 9-10. These pin pairs belong to gate III and II repsectively as shown

in Figure 4. The conductance was measured between pins 1 and 2 when pin 1 was shorted to 9 and 2 was shorted to 10. This parallel connection yielded a figure of merit M of 0.18. Other series connections shown in 4 and 5 also

yielded a high value of M, namely 0.46 and 0.54 repsectively. The series

(experiments, 1, 4 and 5) always will yield a high value of M compared to the

parallel connection (experiment 3) or a single pin-pair connection (experiment

2). This can be explained by the following arguments.

M = (Gy - Gd )/Gd = AG/Gd

AG = conductance increase due to moisture condensation

Gy = wet conductance and Gd = dry conductance

178
When two pin pairs are connected in series, dry conductance decreases, and

because the area of condensation increases, A G increases. Referring to the

circuit model shown in Figure 3, when the two pin pairs are connected in

series, the G^'s of the pin pairs are in series and the G2

's due to moisture

condensation between metal tracks of the pin pairs are in parallel because

the G 2 's are in parallel, AG is high and because the G^'s are in series G^

is low and the value of M is high for a series connection.

When we connect two pin pairs in series a complete electrical isolation

is assumed, and this is not true. When two pin pairs are connected in parallel

AG increases and so does the dry conductance and hence M will not increase.

When two pin pairs are connected in parallel, the M value is high (= 0.18)

compared to a single pin pair 1-2 for the simple reason that there is electrical

interaction between 1-2 and 9-10 and the conductance values 1-2 and 9-10 are

not exactly identical. The figure of merit is high for a series connection

provided one selects two or mere pin pairs having minimal electrical interaction

between them and connects them in series.

CONCLUSION

The a.c. conductance technique is a simple and a powerful method to

determine the presence of moisture in an IC package. The conductance measured

between two pins of a dry package shows a positive slope change with respect

to temperature when the chip is taken through a cooling cycle followed by a heating cycle. When there is moisture present in the package, the slope

changes from positive to negative at four distinct points; two during cooling

cycle and two during heating cycle. The four sharp transitions occur (i)

when water condenses on the chip (ii) when water freezes (iii) when ice melts

and (iv) when water evaporates. The third transition was especially sharp

and could be used as a positive identification of the presence of moisture

in a package. But normally one is interested in the amount of moisture in a package. Hence the temperature at which the conductance slope changes is used

fot moisture determination. A criterion to select a pin pair is given by the

following procedure:

1. Measure conductance between all pin pair combinations.

2. Select the pin pairs which have a low conductance value between them.

3. From the pin pairs selected in step 2, select the pin pairs that

have nearby metal tracks and long parallel metallization runs. A

layout diagram of the chip is necessary.

The pin pairs selected can be connected in series for increased

sensitivity. The pairs to be connected in series are selected as follows:

a. A suitable set of pin pairs has been identified.

b. From the layout diagram, pick out pin pairs which are in different

geographical locations on the chip. Make sure pin pairs are electrically isolated.

c. Connect the pin pairs in series to increase the dry conductance

value

.

179
d. The figure of merit of the connection should be high.

The previous procedures are useful in selecting proper pin pairs and a series mode of connnection which will yield an increased figure of merit.

180
References

1. N.K. Annamalai, and S.M.R. Islam, "Moisture Determination in IC

Packages by Conductance Technique", Proceedings of the 24th International

Reliability Physics Symposium, Anaheim, CA, April 1-3, 1986.

2. N.K. Annamalai, and R.W. Thomas, "Moisture Measurement in CERDIP Packages

by Conductance Technique:, Proceedings of the Moisture and Humidity International

Symposium, Washington, D.C., April 15-18, 1985.

181
CONDUCTANCE &

TEMPERATURE

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4.2 EVALUATION OF A PARASITIC CHARGE-SPREADING TRANSISTOR AS A MOISTURE SENSOR.

Karin Hulten and Orjan Hallberg

RIFA AB S-163 81 STOCKHOLM Sweden

+46-8-757 49 20

ABSTRACT

This paper reports on a practical evaluation of a parasitic charge-spreading

transistor (CST) as a moisture sensor. The device in question has a lateral PNP structure and operates as an MOS p-channel transistor where the gate charge is

built up by charge-spreading over the oxide.

A model is proposed for the dependence of relative humidity (RH) on surface conductivity, which allows extrapolation to low values of RH. This

model is evaluated by experiments using the CST device.

An estimate of the time it takes for water to permeate the bulk of a plastic IC package in different environments is made using the CST.

THE OBJECTIVE OF THIS WORK

This work aims at a better understanding of the parasitic phenomenon and its

dependence on voltage, humidity, temperature and surface cleanliness.

The main purpose of this work was to evaluate the parasitic chargespreading transistor as a humidity sensor in different applications. The device

could possibly be used for monitoring moisture content inside hermetically

sealed encapsulations. Another possible application is to use the CST for controlling hermeticity of the packages, something that so far has been tested

with helium or krypton.

A third area to be investigated was the possibility of using the sensor

for characterization of plastic enclosures, in the case of water penetrating

the package.

THE TEST STRUCTURE AND TEST SPECIMENS

Special test chips were made that contained a lateral PNP structure (CST). They

also contained a normal MQSFET for measuring the field threshold voltage (fig.

1). Some of the testchips had a 10.000 A thick passivation layer on top of the

thermal oxide. The passivation was a phosphosilicate glass (PSG), trading name

Vapox, of sandwich type. The rest had only the 10.000 A thermal oxide.

The devices were enclosed in different encapsulations; plastic, metal

can and ceramic dual-in-line packages (CERDIP).

Figure 1 The CST test structure 189
THEORY Charge spreading /I, 2/

The voltage distribution on the oxide surface near a conductor

by the diffusion eguation:

The solution given by:

3hi y?

V

. 3V

/a It

\Jo erfc

**0 4ot

l •r

can be described

---(1)

---( 2 )

Where: Vo = applied bias a = surface conductivity

q = dielectric capacitance density

x = distance from conductor

t = time

Humidity dependence

The surface conductivity of Silicon Dioxide has been modelled and described

according to the BET theory at this workshop in 1978 by R P Merrett and S P Sim. I'Ll In summary the surface conductivity (s) according to the data in their

report can be described by using the following expressions: (somewhat

simplified)

m=cr/ ( ( 1-r) ( 1+r(c-1 ) )

)

s=am*exp(-1 1 605* .82*( 1- .09m)/T

)

where m= proportional to the amount of adsorbed water

c= exp(0/kT)

a= constant , here 0.0001

r= relative humidity (0-1)

T= temperature (K) 0= difference between the latent heat of evaporation of water and

the heat of adsorption of this water, here 0.035 eV The water adsorption on Si02 is plotted in figure 2 from ref /4/. This S-shaped

curve looks similar to a cumulative density function curve when drawn on a linear scale. By the use of a standard log normal distribution diagram this curve could be straightened out and thus helpful when extrapolating down to low

values of RH. Eguation (4) was plotted in such a diagram to see if the expression

would justify the use of it for linearisation purposes. The result is shown in

figure 3 and it indicates that one should expect linear relations from roughly

60?oRH down to at least 2%RH provided the expression is valid.

Figure 4 shows published data on surface conductivity replotted in a log normal diagram. These experimental findings form the basis of the humidity

dependence used later' for evaluating the moisture sensor readings.

In the literature on the related subject, corrosion, one can find

several moisture models. Exp(a*(RH)2) by Lawson or a/(RH)2.67 by Peck have been

used to linearise the data and to get moisture acceleration factors. /1 6 , 1 5/

.

Ref /5/ reported acceleration factors based upon a collection of moisture test results. In figure 5 that material has been updated with more recent data. The

models are compared in figure 6. Temperature dependence

---(3)

...(4)

In our computer model we have made use of the log-normal behaviour of the RH- dependence and assigned an Arrhenius function for the temperature dependence of

the conductivity at 50?o RH. As the activation energy varies from 0.35 to 0.8 eV depending on the RH /6/ this will result in different slopes in the log normal

diagram. The lower the relative humidity the higher the activation energy. 0.35

eV is close to that of conduction in bulk water (0.34eV). Appendix A gives the

fortran program that makes use of the model in order to estimate the ppm-level

in a hermetic package with a CST device.

190
EXPERIMENTS WITH TEST CHIPS IN CERDIP PACKAGES

The time taken for the applied voltage to spread out on the oxide surface and

eventually turn on the transistor was measured. (To be more precise, it was the

time needed for the leakage current to reach a specified value.)

Humidity dependence

Tests were performed at room temperature with unsealed components in

desiccators with different moisture contents.

12% RH: LiCl.H

2 0; 32% RH: MgCl

2 .6H2 0; 76% RH; NaCl

When the components to be tested had been placed in the vessel, they were left

for 24 hours for equilibrium to be established before any measurements were conducted

.

When measurements were made at room ambient, the occational great

variations in the degree of humidity were monitored using a calibrated

hygrometer

.

The tests at 76% RH had to be abandoned since the turn-on time was too short for accurate measurements.

In fig 7 the turn-on time vs. relative humidity is plotted in a lognormal diagram for a few components. The results agree fairly well with the

model, even though the spread between devices is relatively large.

A normal non-leaking Cerdip contains a total amount of moisture of about 1000 ppmv at 100°C according to mass spectrometric investigations. Taking into account the large surface-to-volume ratio of the ceramic package cavity, which

leads to a large moisture uptake by the cavity walls, the water vapor that is free to absorb on the chip surface is around 200 ppmv /8,9,10/. This

corresponds to approximately 0.4% RH at 22°C and as the pressure inside the die

cavity of a Cerdip is approximately 0.3 atm this is equivalent to 0,2% at 1 atm. For this case, log normal linear extrapolation predicts a turn-on time of

200 hours for an applied voltage of 100 V. No leakage current was detected

however, even though the test chips were kept under constant bias for 500 h. There are two possible explanations for this; either the test structure

is not sensitive enough for such low conductivities, or there exists a limit

below which the model is no longer valid.

Voltage dependence

The voltage dependence was examined at constant humidity and temperature (22 C, 12%). The time elapsed until the leakage current reached -150 nA was measured.

The results are shown in fig 8. The theoretical values can be calculated by keeping one point fixed

(e'.g. 19s, -45 V in fig 8) while adjusting the threshold voltage for best fit. As can be seen, the theoretical and experimental curves are in excellent

agreement. This also seems to be a way to estimate the threshold voltage for parasitic action.

Temperature dependence

A simple test was made in order to see if the CST device would show the same temperature dependence as siyrface conductivity has shown /€/

.

A device was stored at 30 % RH at varying temperatures. The turn-on time was recorded and

is presented in figure 9. The device had vapox passivation. The activation

energy was 0.62 eV that is very close to that reported by Koelmanns at 30% RH /Ref 6 /

.

191
EXPERIMENTS WITH "REAL" DEVICES

Instability mechanisms were reported from lifetesting of a particular circuit.

It was thought to be due to parasitic action, which could be related to an unnecessary opening in the passivation layer. The chip layout is partly shown

in fig 10. This device could have its input bonded to one of two pads in order

to obtain two functions with only one chip. The input is on (a) in the first case and on (b) in the second case. In the circuit of interest the input is on* (a) and (b) is not used.

When the emitter (c) is kept at a low potential, (b) ends up at about

the same level. The distance from the contact-hole (b) to the pnp-transistor

(d) is only about 25 urn and if moisture is present the potential can spread out and possibly turn on the transistor.

According to equation (Z), the turn-on delay has a quadratic dependence

on the distance x. Theoretically, the turn-on should be delayed by a factor 100

if the distance was increased from 25u (b-d) to 250u (c-d).

Comparative tests were carried out on components with and without this

bond-opening. The result is presented in fig 11. As can be seen, the predicted

delay by a factor 100 is not reached but the improvement ( by a factor of 33 using the median value) is clearly shown.

PLASTIC ENCAPSULATION

Diffusion times for water through the bulk of the plastic in different

environments were studied and the acceleration factor between 85C, 85?o RH and

120C, 100?o RH was determined.

Components without PSG

The turn-on time for the test chips in autoclave (120/100) was measured once a day for five days and the components in 85/85-test were monitored continuously

for a period of 3 weeks. The applied bias was -80 V. Components with PSG

It was difficult to obtain any results at all from devices covered with

passivation. The conductivity is greatly reduced by the plastic and the

inversion threshold voltage is higher for components with Vapox, so in order to get any results within reasonable time the applied voltage would have to be

raised above the breakdown voltage of the diodes. The only component that could

be measured had a turn-on time of 80 minutes.

Results

for the components in saturated autoclave, water reached the chip surface after

a little more than one day. The time needed for those tested in 85/85 was about

3 weeks. This gives the acceleration factor A.

A = ( 21 days)/ (1.6 days)= 13

In one case, the plastic seemed to have been displaced from the chip surface.

This gave rise to an increase in sheet conductivity with a factor of 80.

192
SENSOR STABILITY

In order to check the reproducibility, the test chips were measured directly

after removal of the lid and then remeasured (after 30 seconds, 30 minutes, 1 day and 3 days). It was concluded that 30 seconds was too short a time for the

outspreading surface potential to relax, and this resulted in shorter and

shorter turn-on times. Waiting for 30 minutes or 1 day between measurements

gave satisfactory reproducibility. The values differed about 10-20?o. After 3 days the turn-on time for the chips kept at 12?o RH was increased by a factor of 3 compared with the first measurements, while the turn-on time for those kept at room ambient did not change. This was somewhat

unexpected and needed of more thorough examination.

Cerdips

Directly after removal of the lid, Cerdips were extremely sensitive to variations in relative humidity, but this sensitivity became less and less

pronounced with time.

This "aging process" turned out to be strongly dependent upon humidity,

i.e. storage in a dry atmosphere had an accelerating effect (fig 12).

A certain difference between chips with and without Vapox was noted in the sense that devices with Vapox tended to have a slower loss of sensitivity.

This might be due to the hygroscopic nature of Vapox.

Metal cans Devices encapsulated in metal cans seemed to be completely insensitive to moisture. They showed no tendency whatsoever to leakage. This experience was

not new, the same behavior had been observed before in connection with otheT measurements / 1 2/

.

Washing for a few seconds in dilute HF (10 sec. 1?o HF ) restored the

surface of the chip and the behavior was then the same as for Cerdips.

Why a dry atmosphere increases the contamination rate

The decrease in sensitivity is thought to be due to organic vapor in the air.

These impurities make the surface hydrophobic, which means that condensed water molecules on the surface form small droplets instead of a continuous film, thus

reducing the surface-charge spreading to a great extent.

Tamai et.al./13/ have studied the effect of water vapor on the

contamination of metallic oxide surfaces, using the contact angle of water as a criterion for surface cleanliness. Their results show that the contamination

rates of the clean surfaces are reduced by the coexisting water vapor (fig 13).

This can be explained by the fact that the organic contamination is to be regarded as an adsorption process, and not just due to material "falling out" onto a surface / 1 4/

.

An oxide surface is often strongly hydrophilic, which implies that it has a preferential bonding to water. This in turn means that the organic

contaminants should adsorb onto the surface competing with water molecules.

Consequently, a dry atmosphere means "free access" to the adsorption sites.

The difference between cerdips and metal cans

Metal encapsulated chips appear to be insensitive to moisture, while those in

Cerdips are very sensitive. This would mean that some step in the Cerdip

encapsulation process radically affects the surface.

An efficient method for cleaning surfaces that have been exposed to organic vapor involves heating to 400-500 C in an oxidizing atmosphere. Cerdips

are sealed in dry artificial air and the temperature during this process is around 450 C. The surface can thus be cleaned as a result of the sealing

process /9/.

When sealing metal cans, the temperature is much lower and the

atmosphere is non-oxidizing, so the surface is still contaminated after the
previous encapsulation process. This explains the low sensitivity of metal can encapsulated CST devices.

CONCLUSIONS

CST can be used for leakage testing of Cerdips immediately after the

encapsulation when the surface is clean,. Leakage testing at a later stage may give unreliable results because of the atmospheric contamination that will take

place if the package is not hermetic. This will result in a hydrophobic surface

and the CST will become less sensitive.

It is difficult to use the CST-sensor in order to determin the exact moisture

content inside a hermetic package because of the wide spreading between

individual devices. Sorting out components with a water vapor content that

exceeds a critical value of, say, 5000 ppmv may however be possible though.

Measurements can be made in sample tests where each component must be

calibrated in a desiccator with known relative humidity. (Appendix A)

If different plastic package materials are to be evaluated regarding moisture

resistance, the CST seems to be a good indicator of water penetrating the

package. Acceleration factors for TH-studies (Temperature-Humidity) can also be

determined. The field-inversion threshold voltage must be kept low in this

application (no Vapox) because of the great reduction in surface conductivity

when the plastic adheres well to the die.

ACKNOWLEDGEMENTS

This work was sponsored by the Swedish National Microelectronics Program

Quality assurance project NMP-Q in cooperation with RIFA AB.

APPENDIX A. Program

hermetically sealed devices

for calculating the moisture content inside

PROGPAM FOR CALCULATING THE MOISTURE CONTENT INSIDE HERMETICALLY SEALED DEVICES (EXPRESSED IN PPMV)

EAD VALUES FOR THRESHOLD VOLTAGE, OPERATING VOLTAGE, TURN-ON TIME

URING OPERATION, OPERATING TEMPERATURE, TEST VOLTAGE, TURN-ON TIME

N TEST, TEST TEMPERATURE, ACTIVATION ENERGY FOR TEMPERATURE DEPENENCE, RELATIVE HUMIDITY IN TEST. WRITE!*,*) 'THRESHOLD VOLTAGE

READ! * • ) VTH WRITE ( * , • ) ‘OP. VOLTAGE, TURN-ON TIME DURING OP.. OP . TEMPERATURE

READ ( *

, * ) VB0.TIME0, TEMPO

WRITE! *,*) TEST VOLTAGE, TURN-ON TIME IN TEST, TEST TEMPERATURE

READ ( * , * ) VB1 .TIME! .TEMPI WRITE!*, •) ACTIVATION ENERGY ( EV

)

'

READ ( *

, * ) EA WRITE!*,*) 'RELATIVE HUMIDITY IN TEST

READ!*,*) RH

1020

304050

C

C THE ZERO OF THE ERROR FUNCTION IS COMPUTED

C RESO-ERFX(VTH.VBO)

RES1-ERFX! VTH.VB1

)

B7* ( RES0/RES 1 )**2

C

C THE ACCELERATION FACTOR FOR INCREASED HUMIDITY IS CALCULATED

100

B1-RHACC(RH, TEMPI)

C C

C THE ACCKLERATIONFACTOR FOR INCREASED TEMPERATURE IS COMPUTED

C B3-TEMPACC (TEMPO, TEMPI ,EA) B8«TIME1/TXME0*B3*B1/B7 c

C

C AN APPROXIMATION WITH THE NORMAL DISTRIBUTION FUNCTION Q(X) IS DONE

C AND THE ARGUMENT X IS CALCULATED.

C B9*0FUNC( TEMPO, B0) RES*B9* 1C6/30O*9.8E8*EXP(-5235/( 27 3+TEMP0 )

)

CC C

C PRINT THE PPMV-VALUE

WRITE! *,*) ‘THE PPMV-VALUE X8 ‘ * XNT(RES)

END 194

FUNCTION ERFX(VTH.VB)

ERF--1-VTH/VB SLASK=ERFNOLL( XX ,£RF

)

IF! SLASH. GT.0) GO TO 10 UP=ERF- . 04 DOWN=XX

GO TO 20 UP=XX DOWN=ERF- .04 IF l ABS! UP-DOWN) .LT. 0.005) GO TO 50 XX= ( UP-*- DOWN ) /2 SLASK=ERFNOLL ( XX , ERF

)

IF! SLASH. LT.0) GO TO 30 UP=XX GO TO 40 DOWN=XX

GO TO 20 ERFX-=XX RETURN

END FUNCTION RHACC ( RH , TEMP

)

SLASK=2. 3-TEMP/75

R= 0 IF! RH .LE. 50 ) GO TO 100 R=RH

RH= 1 00-R 0S=SQR7 ( LOG ( 1E4/RH**2)

)

ZZ=OS- (2. 3075+0. 2706* OS )/( 1 »0 . 9923*OS+0 . 0448* ( OS* * 2 )

9

IF! R.GT. 50 ) ZZ=-ZZ RHACC= 1 0* * ( -SLASH* ZZ

)

RETURN

END FUNCTION TEMPACC ( TO , T 1

, EA

)

TEMPACC=EXP( 1 1 605* EA* t 1/( 27 3+TO ) - l/( 27 3+T1 ) )

)

RETURN

END FUNCTION QFUNC(TO.B)

SS=2. 3-T0/75 XS=-LOG10(B)/SS

YS= 1 /

!

1 +0 . 3327*XS

)

QFUNC=SQRT( 2/3 .14 16 ) /2*EXP( - ( XS* *

( .9373*(YS**3) ) ) RETURN

END

2 ) /2 ) * l .4 36 2* YS- . 1202MYS**

FUNCTION ERFNOI.L! U , V

)

ERFNOLL=!U**9/216-U**7/42+U**5/10-U**3/3*U)*2/SQRT( 3 . 1410)-- RETURN

END
References

1. Schroen, W., "Failure Analysis of Surface Inversion", Proceedings 11: th Reliability Physics Symposium, 1973.

2. Merrett,R.P. , Sim,S.P., "Assessment of the Use of Measurement of Surface

Conductivity as a Means of Determining Moisture Content of Hermetic

Semiconductor Encapsulations", ARPA/NBS Workshop V, Moisture Measurement

Technology for Hermetic Semiconductor Devices, 1978.

3. Kawasaki, K. ,Hackerman,N. , "On the Variation of Surface Conduction Current

of Porous Vycor Glass by the Adsorption of Water Vapor", Surface Science

10, 1968.

4. Ammons, J.M. , Hoff ,G.R. ,Kovac,M. , "The Physisorption of water onto Integrated

Circuit Package Components", RADC/NBS Workshop, Moisture Mearurement and

Control for Semiconductor Devices, III, 1984.

5. Hallberg,0., "Acceleration Factors for Temperature-Humidity Testing of Almetallized Semiconductors", SINTOM, 1979.

6. KoelmanSjH., "Metallization Corrosior in Silicon Devices by Moistureinduced Electrolysis", Proc. 12th Reliability Physics Symposium, 1974.

7. Sbar ,N.L . ,Kozakiewicz , R . P . , "New Acceleration Factors for Temperature,

Humidity, Bias Testing", Proc. 16th Reliability Physics Symposium, 1978.

8. Finn, J.B. , Fong, V. , "Recent Advances in A^O^ "in-situ" Moisture Monitoring

Chips for Cerdip Package Applications", Proc. 18th Reliability Physics

Symposium, 1980.

9. White ,M. L ., Sammons , R . E . , "A Procedure for Preparing Hermetic Packages with

known Moisture Levels", NBS/RADC Workshop Moisture Measurement Technology

for Hermetic Semiconductor Devices , II , 1982

.

10. Lowry, R. K. , Miller, L. A. , Jonas, A. ,W. , Bird, J.M. , "Characteristics of a Surface

Conductivity Moisture Monitor for Hermetic Integrated Circuit Packages",

Proc. 17th Reliability Physics Symposium, 1 979

.

11. Morrison, S. R. , The Chemical Physics of Surfaces, Plenum Press NY,

London, cop. 1977.

12 Report RIFA Document No M/Z 83-39.

13. Tamai,Y. ,Matonnaga,T. , Suzuki, K.,"The Effect of Water Vapor on Contamination of Metallic Oxide Surfaces", Bulletin of the Chemical Society

of Japan, vol.50 (7), 1977.

14. White, M.L., "The Detection and Control of Organic Contaminants on Surfaces", Clean Surfaces, G.Goldfinger,ed. , Marcel Dekker Inc., NY 1970.

13. Lawson R.W., "A review of the status of plastic encapsulated semiconductor

component reliability", Br Telecom Technol J Vol 2 No 2 April 1984.

16. Peck, D.S., "Comprehensive Model for Humidity Testing Correlation", IEEE IRPS 1986, pp 44-30.

195
Angstroms

Water

Adsorbed

PPmV water (x 1 0

*

3

)

Figure 2 Water adsorption

on S i

0

2 /Ref 4/

RH%

Figure 4 Data from Keelmans 76/

RH%

Figure 3 Calculated according to eqn (4)

Key RHV. 2046-55

60-65

+ 70-76 A 80-85

90-96 O 98-100

Figure 5 Corrosion acceleration factors from /5/ and other sources

A
Turn-on time(s)

Turn-on time (s)

Figure 8 Bias (-V)

Turn-on time vs bias at 22C/12%RH

Temperature dependence of turn-on time

Figure 10 A pnp transistor

with CST properties

Cdf (%)

Distribution of values

Loss of sensitivity at different ambients Change of contact angle by time. From ref / 1 3

/
4.3 MOISTURE MEASUREMENT BY AC-BIASED

SURFACE CONDUCTIVITY SENSORS

Didier Kane and Benjamin A. Moore

Rome Air Development Center (R3RE)

Griffiss AFB NY 13441-5700

(315) 330-4055

* Dr. Kane is a contractor from Clarkson University, Potsdam, NY

INTRODUCTION

Tomorrow's military and space electronic devices will cost

thousands of dollars, not to speak of the overall cost of the systems in

which they will be inserted, for which no risk of failure will be accepted.

Even though serious progress have been made in terms of moisture

control, with the implementation of Method 1018 in MIL STD 883C, none of

the techniques currently used can provide the insurance that a precise part

selected for a given system will never experience a moisture related

failure.

Among the non-destructive techniques, aluminum oxide sensors

could be an answer for hybrid modules in which volume is not a major

requirement, but the presence of an additional die will not be possible in

most packages. The conductance technique using the die as a moisture

sensor seems to be an ideal solution, but further evaluations are still

necessary for determining the die-to-die reproducibility and sensitivity.

Another possible solution that would probably be less dependent on the

nature of the die would be the integration of surface conductivity sensing

schemes as electrical test' structures on the side of the die. This could

be done with no additional mask, but would require four 1/0, which might

not be viable for some packaging approaches.

At the present time, RADC is still evaluating surface conductivity

moisture sensing chips whose design might later be integrated on any die.

Such sensors have been shown to be accurate but not reproducible when used

with DC bias (1). The purpose of this paper is to present recent

improvements brought to this technique by AC bias and to point out the

importance of the adsorption on the cavity walls in any "dew-point"

measurement technique.

198
DESCRIPTION OF THE TECHNIQUE

The sensor is made of an interdigited aluminum stripe pattern

deposited on an insulating material (thermally grown thick silicon oxide)

As shown in Figures 1 and 2, two different test structures have been used

in this study; the main differences between these two structures are the

shape and length of the aluminum electrodes and the existence of a diode on the later die itself, which enables more accurate temperature measurement.

The sensing die is mounted in the package to be tested. The method

consists of heating the package up to 100 C and maintaining this

temperature for a given period of time to desorb the moisture fixed on the cavity walls in the same conditions as in a mass spectrometer. The

package is then slowly cooled down, while the inter-electrode leakage

current of the biased sensor is recorded versus temperature. Figure 3

represents the response curve that can be expected for an hermetically

sealed package. It has been shown that the "dew-point" corresponds to the

onset of conduction (change in slope), and not to the top of the peak (2).

If the sealing pressure and temperature are known, some charts based on the

application of the ideal gas law to the partial pressure of water in the

package usually allow to convert the electrical "dew-point" into a moisture

content in parts per million. However, these charts do not take into

account the adsorption of moisture on the walls which makes it impossible

to speak of a real dew-point in a closed package, and a model presented

later in this paper will show how modified ideal gas law equations that

take this effect into account can be writen.

Figure 1

199
ELECTRICAL RESPONSE OF SENSOR

I (INTERELECTRODE LEAKAGE CURRENT)

o = ONSET OF CONOUCTION

DIRECTLY RELATED TO MOISTURE CONTENT

100 +60 +20 T -20 TEMPERATURE RECORDED WHILE COOLING PACKAGE

-> T(?C)

Figure 3

200
DESCRIPTION OF THE TEST SETUP

The test setup built at RADC includes a test chamber and an electrical measurement circuit. The test chamber, made of aluminum, is

connected to a mechanical pump, allowing a primary vacuum (20 Pa. ) inside

the chamber. This design eliminates any possible condensation on the

outside of the package, and minimizes the heat exchange with the

surrounding medium. The pump is used for measurement on hermetically

sealed packages only.

For calibrations and physical studies on delidded packages, the

chamber also has an inlet and outlet for gas atmospheres. The inlet is

connected to the RGA calibration system at RADC, which permits a continuous

flow of controlled, humid gas in the chamber. The chamber has a removable

top (fixed by 0-ring), hermetic connectors and a power feed-through. A

schematic representation of the test chamber is given in Figures 4 and 5.

HI?

... ni,. %

:.in in ~~

D.U.T.

r

r n'n

r i frR l

1 1 l_l

WATER * INLET

>RRPTP

,^-j TEST CHAMBER

THREADED HDI FS

THERMOCOAX HEATER

PELTIER COOLERS

TF'TP—

|

WATER * OUTLET

CO? A! IX COOLING

Figure 4

201
SCHEMATIC REPRESENTATION OE TEST SETUP

Figure 5

Thermoelectric heat pumps have been chosen for cooling the

packages. The system uses : - one heat pump module (MELCOR CP 1.4-127-06L) biased with 11V-4A

- three heat pumps in series biased with 11V-2.75V

(MELCOR CP 1.4-31-06L + CP 1.4-71-06L + CP 1.4-125-06L)

Two sample holders have been designed for use with different

kinds of packages. The first one is a copper cylinder with a groove in

which a thermocoaxial heater (SODERN 1 Nc Ac 20) has been welded at both

ends of the heater to avoid moisture related problems. Heat from the hot

surface of the heat pumps is evacuated through a water circulation box.

During the experiments, circulating water has been replaced by ethylene

glycol, allowing a more efficient cooling. A cold finger on top of this

copper piece allows a preferential cooling just underneath the sensor.

This sample holder was used with TO-8 packages. An aluminum bracket with a large threaded hole on top was placed over the sample. A screw was inserted in this threaded hole. At the end of this screw, several heads

could be adapted, according to the kind of device under test. These heads

were made so that they could fit exactly around the top of the package to

be tested. These heads have large openings on the sides, which allow a flow of gas to circulate freely around the sensor when a delidded package

is being tested. A nichrome wire is wrapped around these heads and biased

so that a thermal gradient of 1 to 2 C is always maintained between the top

and the bottom of the DUT. This feature, together with the cold finger

make sure that the sensor is always the coldest point of the package. Some

T0-5 packages that had an insulating glass at the bottom of the package

202
between the leads, had to be tested. This glass was a very bad thermal

conductor, and the previous sample holder could not be used : too much

heat was lost in the thickness of the material, and the sample could not be

properly cooled. A new sample holder was therefore designed. Its shape was

a very thin copper pyramid with a ring (three quaters of a circle long) on top. This allowed cooling from the metal ring at the bottom side of the

DUT. Moreover, some tiny holes in which were inserted the leads that were

not used for the electrical measurements provided a second cooling

source. A second ni chrome resistor was wrapped around the appropriate

screw-head in order to heat the DUT up to 100 C and to maintain it at this

temperature at the beginning of each experiment. The screw-heads also

permitted retention of a good thermal contact between the DUT and the

sample holder. This was improved by the deposition of a thin layer of

silicon grease between each surface involved in the cooling process.

An automated thermal cycling system, identical to the one described in reference 1, has been added to the test setup.

ELECTRICAL MEASUREMENT CIRCUIT

Some Auger analysis has been initially performed on sensors previously biased with 40 VDC and exposed to more than 50 temperature

cycles in the range +110 C to -30 C, in a highly humid atmosphere, which

permitted :

- verifying the presence of alumina on the anode, resulting from the

oxidation of the electrode after electrolysis of the adsorbed water.

- confirming the presence of aluminum on top of the silicon oxide in the

inter-electrode space.

- confirming the choice of AC bias instead of DC bias for a better

reproducibility in the mo.isture measurement.

The non-reproducibility in the electrical response of a sensor mounted in a hermetic package and exposed to thermal cycling under DC bias

had previously been observed, and may be explained (1) by a modification in

the composition of the silicon oxide upper layers.

Consequently, it was decided to use a Lock-In-Amplifier (LIA)

whose internal generator would be used for biasing the DUT with a low

voltage AC signal and would also be used as a reference signal. A very

sensitive digital microvoltmeter was used at the output of the LIA for

detecting the voltage across a resistor in series with the DUT.

Optimizing the various settings on the LIA was a necessary

condition in order to reach the required sensitivity in the measurements.

203
It implied selecting the appropriate time delay, bandwith, filters, and

frequency. After many attempts, a frequency of 37.1 KHz was selected for

providing the best signal to noise ratio. The choice of a high frequency

was aimed at decreasing the 1/f noise, to which the device was very

sensiti ve.

Figure 6 gives a schematic representation of the measurement

ci rcuit.

pnrniip

( f'n n in)

Lor K r» flririirirn

( / n n dm ) loo,..v l _

WAirni.

VOI I IIFTFR

XY

R ECORDf R

HlfiiTOi. lOUtif TF H

Figure 6

204
EXPERIMENTAL RESULTS

Successful results were obtained with biased sensors

eutecti cal ly mounted in delidded TO cans and exposed to various controlled

atmospheres. These sensors were AC biased (100 mV, 37.1 KHz), initially

heated up to 100 C and maintained at this temperature for several hours,

and then slowly cooled down to around -30 C. The temperature was measured

with a diode eutectically mounted nearby the sensor. Very good accuracy

could therefore not be expected because of the unknown thermal gradient

between the temperature sensing diode and the sensor itself. The output

signal of the Lock-In-Amplifier was monitored versus temperature during the

cooling of the package.

The first experiment did consist of looking at the response of a sensor exposed to the room dew-point (controlled in flow conditions by a General Eastern mirror hygrometer). An onset of conduction (change in

slope) could be very clearly detected within 1.5 degrees of the room dew-point. The experiment was repeatable. Each time, the detected

dew-point temperature was slightly higher than the value indicated by the

hygrometer. This had also been observed previously with DC bias. It might

have two causes : - The thermal gradient in the package

_ The need for a certain amount of water to be adsorbed before a conduction

can be electrically detected. This amount could be around three

monolayers for ionic conduction, and probably a little less than one monolayer for electronic conduction. A theoretical study on conduction

mechanisms in adsorbed layers between AC biased electrodes would be

necessary for confirming this hypothesis.

The most important series of experiments consisted of studying

the sensor response to a wide range of known dew-frost points.

The test chamber was therefore connected to a gas generator that allows

a flow of controlled humid gas to circulate. Several dew-frost points were generated in the range [ 0 C to -30 C ] and the response of the sensor

was recorded each time in the same conditions as for the first experiment

described above. The results are sumnarized in Figure 7.

205
V (..'*>) nrri\»i^nV D**i Pom)*, t vi 111* Ch»mK«r

_ : - f. ? C.

io s •- r u „ . r

/ V

• • • • ••-•-* 1 • • • •

- —4- - • i - > t (:c)

<• * - l » -** .)0 - 11. ->«

F. X TE RiMfNTfiL c niinn n r ions

Figure 7

Several remarks can be made on this Figure :

1)

The intensity of the leakage current total variation decreases with dewfrost point temperature. This means that the lower the moisture

content, the lower the conduction. Therefore, the presence of water in

the liquid phase and its abundance strongly affect AC conduction as it

did for DC bias. This seems logical as water can be considered as a medium in which electronic and ionic conduction can occur, and as a source of ions.

2)

There is an experimental limit (approximately -25 C) below which no conduction can be detected. This limit is the same as the one observed

with DC bias. A physical model for explaining the existence of this

limit has been proposed elsewhere (1).

3)

It is difficult to correlate precisely the temperature at which an onset

of conduction was detected and the generated dew-point temperature

because of the thermal gradient in the package. Therefore it is

important to be have to measure accurately the temperature on the sensor

itself, if one wants to make a quantitative measurement.

206
As a possible answer to this problem, 13 new sensors were sent to

us (courtesy of Harris Corporation), in which a diode was integrated in the

chip itself. 12 of them passed a detailed visual inspection while 1 revealed a shortage between al umi num metal lines in one location. These

parts were sent to Raytheon Co. for mounting (eutectic Au-Si), wire

bonding and controlled sealing. The 12 "good" parts were divided in 4

groups of 3 devices, each group being sealed with a different moisture

level (1,000 ppmv; 3,000 ppmv; 5000 ppmv and 8,000 ppmv). When the parts came back, it appeared that it was impossible to cool them down enough to

reach a condensation point (the notion of dew-point is a closed package and

the influence on the adsorption on the walls will be discussed later in

this report), so that no measurement were possible. Some of them were

open, which revealed that the dies were not properly attached. A failure

analysis revealed that, even though a preform had be used, the eutectic did

not stick to the die because its back surface had been previously etched.

None of these dies could therefore be used, which would have given an answer to the accuracy of this measurement method.

207
PHYSICAL MODEL FOR QUANTIFYING THE EFFECT OF ADSORPTION ON THE CAVITY WALLS

The concept of dew-point has often been improperly used when

reporting moisture measurements based on in-situ devices for which a cooling process is necessary. This is because one usually neglects the

effect of adsorption on the cavity walls. Most tables and graphs giving a direct correlation between a dew-point temperature and a moisture content

expressed in ppmv are based on the strict application of the ideal gas law.

The intent of the following model is to develop a modified ideal gas law

taht takes into account the adsorption on the cavity walls during the

cooling of the package. This is very fmportant when one tries to compare

results obtained by two di fferentanalyti cal techniques, in which this

contribution can be very different.

The ideal gas law applied to the partial pressure of water gives :

P V = n R T

where P is the partial pressure of water, V the volume of the package, n the total number of moles, R the constant of gas and T the temperature.

If one takes the adsorption into account, it becomes :

P V = ( n, co - n*ls) R T

where n is the total number of moles of free water vapor at 100 C

(neglecting what is adsorbed at this temperature) and n , the number of

moles of water adsorbed on the walls at any temperatue T below 100 C.

The determination of n is the critical parameter that will enable a comparison between what can be measured by a technique working at 100 C

(RGA, IR), a technique working at room temperature (Panametrics sensors) or

a "dew-point" technique (surface conductivity, conductance technique). It

is strongly dependant upon the surface areas of the materials involved, the

surface to volume ratio and the thermal gradient in the package.

In the case of the samples on which the experimental study was performed,

if one neglects the amount of water adsorbed on the sensor itself as compared

to what is adsorbed on the gold-plated cavity walls during the cooling process,

and the existence of a thermal gradient aimed at condensing moisture first

on the sensor, n ^ can be determined by the BET model.

208
For this extremely simplified model, the number of water molecules

adsorbed on the cavity walls at a given temperature and relative humidity

can therefore be written :

r

n ad* = n c ( — ) * 1-r

exp(07RT)

) l+r[-l + exp(|2f/RT)]

where JZf = El-El is the difference between the heat of evaporation of water

and the heat of adsorption of water vapor on the adsorbate.

n 0 is the number of water molecules necessary to cover the cavity internal

surfaces with one monolayer.

nc , El and El strongly depend on the materials inside the package (plating

of the cavity walls, organic adhesives....), but can be found in literature

for almost any material.

r is the relative humidity defined by : r = —

P°

Where P is the partial pressure of water at a given temperature, and P° the

saturating vapor pressure of water at this temperature.

P can be determined by the Clapeyron equation :

373 AHvap 1 1 P°= Pseal ( ) * exp [ - - ( ) ]

Tseal R 373 T

P.cc * V

n ioo is defined by the ideal gas law : n * co = 373 * R

where P, cc = Ptotal at 100 C * Xppmv * 10'- &

This model has been applied to gold plated TO-8 packages; the

results are summarized in the following table.

209
X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X ,—. X X X X X X X X X X X X (

.O ID no X X X X X X 2: CD i X CJ X CJ X X X X 1—J

TO • f—1 X X X CJ X CJ X X O CD M— cn X X X X X X a. -t-J * no X ru X no X X X X 1 ED TD CD X X -—1 X r^_ X ru X X cl O X 1 X X X X X ljj EE EE 1 1 X X 1 X 1 X 4- X X a O no X X X X X X O CD X X X X X X - TD X X X X X X l—l X X x X X X X X X X X X X X X X X X X x X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X ESI X X X X X X i

no X X X X X

xK :x > _J X X X X X x H jO X X X X X X O cn X X X X X X CL td CD X CJ X CJ X CJ X CJ X X 1 CD CD X X X X X X 3: 4-J X m X X X X X 1 _U ZD 1—i X aj X cn X no X UD X X a CL no X X X X X X £ CD X 1t X 1 X ! X ..J- X X O TO x X X X X X O h-H X X X X X X - x X X X X X x X X X X X X X X X X X X X X X X X X X X X X X X x X X X X x X X X X X X X X X X X X X X X X X X X X X X X X X X X X i— X X X X X * EX X X X X X X LJJ X X X X X X L~ X X X X X X EX LJ ^ v X X X X X X O > X X X X X X CJ O EE X X X X 0 X X O CL X X 0 X 0 X 0 X X LLi CL X 0 X 0 X 0 X 0 X X CC X 0 X 0 X 0 X 0 X X ZD 4-J C X 0 X no X m X ^-H X X l

no * rH X rH X X X X X CD X X X X X X l—\ X X X X X X O X X X X X X >: X X X X X * X X X X X * X X X X X * X X X x X X X X X X X X X X X X X X X X X X

210
RECOMMENDATIONS

The following recommendations can be made at the end of this

study :

Surface conductivity moisture sensing chips can be used for performing

measurements of any hermetic package moisture content provided that all

the conditions listed below are fully satisfied :

- The chip must include a temperature sensing diode

- The die-attach material must be a good thermal conductor, and must

not adsorbe more moisture than the other materials in the cavity.

- AC bias is required in order to avoid reproducibility problems.

- The bias frequency and voltage must be optimized according to the

measurement circuit used. The use of a Lock-In-Amplifier is

advised for obtaining the required sensitivity ( signal to noise

ratio ).

- A thermal gradient must be maintained in the package, so that

condensation can be assumed to occur first on the sensor.

- The adsorption properties of all the materials used in the package

(plating, lid....) must be known precisely, if one wants to make a quantitative measurement, and correlate this result to other

techniques such as RGA, for which the thermal conditions are different.

- A precise knowledge of the sealing conditions ( temperature,

pressure ) is also necessary.

211
CONCLUSIONS 1)

A new, original test setup has been developed at RADC, that allows

measuring non-dest ructi vely the water vapor content of any integrated

circuit hermetic package. This system allows working either in vacuum

for measurements on hermetically sealed parts, or in a controlled humid

atmosphere, for calibrations on delidded parts.

2)

Calibrations have been succesfully performed on AC biased interdigited

surface conductivity moisture sensors eutectically mounted in delidded

TO- packages, for moisture levels corresponding to the range [ 0 C to

-25 C ] in "dew-frost" points.

3)

A physical model has been developed that allows correlating the

electrical "dew-point" measured by an in-situ surface conductivity

sensor to the actual moisture content as measured by a mass spectrometer. This model is a modification of the ideal gas law

applied to the partial pressure of water in the package, that takes

into account the adsorption on the cavity walls during the cooling

process. However, the accuracy of this model would be improved if one could quantify the influence of the thermal gradient in the package.

4) Several recommendations have been users to become aware of this

requirements and limitations.

made that should allow potential

very promissing technique of its

212
REFERENCES :

(1) Recent Developments on Moisture Measurement by Surface Conductivity

sensors Didier Kane and Michel Brizoux

Proceedings of the 1986 International Reliability Physics Symposium

(2) Physical Characterization of Surface Conductivity Sensors in the Aim

of an Absolute Moisture Measurement in Electronic Components

Didier Kane, Robert Gauthier, Michel Brizoux and Jacky Perdrigeat

Proceedings of the 1984 Electronic Components Conference

213
4.4 TUNABLE DIODE LASER SYSTEMS FOR MOISTURE

MEASUREMENT IN INTEGRATED CIRCUIT PACKAGES

D. Wall, J. Sproul, and A. Mantz

Spectra-Physics, Inc., Laser Analytics Division

25 Wiggins Avenue

Bedford, MA 01730

Tunable diode laser techniques for moisture

determination in IC packages developed by

Mucha (Bell Labs, 1981) have been implemented

in commercially available systems. This paper

will discuss details of operation, system

calibration, and performance specifications.

Comparisons to existing mass spectrometric

techniques and instrumentation will be drawn.

Finally, data will be presented showing

correlations between systems and techniques

using prepared ‘package standards.

214
Since the last RADC/NBS International Workshop on Moisture

Measurement and Control for Microelectronics held in 1983, three prototype

semi-automated tunable diode laser systems have been developed and

constructed with two systems delivered to beta sites for evaluation. The

measurement principles are those developed and described by J. Mucha

(1-3). These systems are designed to be automated and except for loading

of packages and puncturing operation, the systems are fully automated

including long-term frequency stabilization (2) of the laser to insure

that the diode laser is operating at the peak of the water line.

In this paper we describe the results from two extensive measurement

programs utilizing one system installed at National Semiconductor (NSC)

and one at Laser Analytics. In these experiments we attempted to

statistically characterize the system performance. The first experiments

were oriented toward establishing a correlation between Mass Spectroscopic

results, measured under the method (MIL-STD Method 1018.2), and the

tunable diode laser results obtained utilizing the method developed by J.

Mucha (1). These results were obtained on production packages which were

collected from production runs.

In the second experimental program, we performed a "round robin"

experiment in which two independent sets of diode laser measurements were

compared with Mass Spectrometric results measured by the MIL-STD Method

1018.2 Method. In this experiment large numbers of packages were sealed

in a carefully controlled atmosphere in which humidity conditions were

carefully adjusted and monitored.

CORRELATION EXPERIMENT RESULTS

Packages manufactured by National Semiconductor Corporation (NSC)

were used in this experiment. Package sizes included T0-3 in which

215
fifteen packages were measured by the Mass Spectrometric (MS) method and

fifteen packages were measured utilizing the tunable diode laser (TDL)

system at NSC. Thirty-nine TO-5 packages were measured by the MS, and

thirty-nine were measured by the TDL. For TO-46 packages, sixteen were

measured by TDL, and ten were measured by MS.

The results are presented for each package size in Figures 1-3. In

each figure the number of packages measured in a concentration range is

plotted on the vertical axis, while the horizontal scale presents H^O

concentration. The width of each horizontal cell (in ppm) is presented in

each figure. For example, in Figure 3 we present results for a TO-46

package, each cell width (vertical bar) is 500 ppm wide, and we measured

three packages in the region 11500 to 12000 ppm utilizing the TDL method.

TDL results are plotted with solid histograms while MS results are plotted

utilizing open bars.

Figure 4 contains a weighted least squares fit to the TDL data

presented in Figures 1-3. Error bars for each package size are presented

as one standard deviation ( 1 c5 ) . The dashed curves represent 1CP limits

on the slope and intercept values determined by fitting the data. Each

package size is represented by its internal total surface area to internal

volume ratio.

Figure 5 shows the Mass Spectroscopic results taken from Figures 1-3

and plotted in a similar manner.

Table I summarizes the surface area/volume ratio for each package

type, accurately calculated from NSC, package drawings, and the mean value

of the measured concentration for each can type. These data are obtained

from the data in Figures 1-3, as well as the numerical value for one

standard deviation for both TDL and MS methods. Finally, calculated

216
concentrations for each can type are calculated from Figure 4 for TDL

results and from Figure 5 for MS results. The calculated concentrations

are presented in Table I.

From Table I it can be seen that, while TDL and MS results differ

significantly for different can types, the standard deviations are

essentially identical. This suggests that the error bars are determined

from the statistical variability in the samples, or the two methods have

similar error limits. The results from the second experiment will provide

more insight to this point.

CORRELATION EXPERIMENT ANALYSIS

Referring to Figures 4 and 5, it appears that both TDL and MS results

when plotted versus internal surface area to volume ratio are linear with

different sensitivity to surface area to volume ratio (slope). In order

to further analyze these results, we plotted the ratio of TDL to MS

results for each package type as seen in Figure 6.

In relating our experiment to previous published measurements on similar metal packages, we utilized the results of the 1981 RADC TO-18

Round Robin which are included in Table II. In the 1981 Round Robin test

four independent Mass Spectrometric measurements were reported as well as infrared measurements made with the package at 100C or 25C. Since our data were run with 100C conditions, we selected only the high temperature

data for comparison.

Table III contains a summary of TDL/MS ratio results. MS values were

selected to be the average value of the four reported MS measurements for

a particular moisture concentration.

217
Figure 7 shows that the TDL/MS ratio, from the 1981 Round Robin

results, is essentially constant over the entire range of concentrations

tested* These data are the same data set presented in Table III, but

presented in a graphical format.

From Figure 7 we conclude that, for a given package type, the TDL/MS

ratio, once measured, is a constant which also tells us that a definite

fixed correlation exists between the TDL and MS results for each package

type.

Taking this analysis one step further, we calculated an internal

surface area to volume ratio for a TO-18 package and plotted the Round

Robin results (labelled Bell Labs TO-18) in Figure 6. The error bar on

the Bell Labs TO-18 data point reflects the one standard deviation value

of the ratio. From the least squares fit data, we calculated a TO-18

value (labelled National Semiconductor TO-18) which is well within the

error bars for the Round Robin data, and which fits the weighted least

squares fit function for TO series cans very well.

TUNABLE DIODE LASER (TDL) AND MASS SPECTROSCOPIC (MS) EXPERIMENT RESULTS

The purpose of this experiment was to produce a large number (3000)

of standard hermetically sealed IC packages (T0-3, -5, -18, and -46) under

known and well-controlled humidity conditions for subsequent testing

utilizing both TDL and MS methods. The description of the procedure for

filling the packages is described as follows:

1 . The Moisture Generator

A variable rate moisture generator was designed and

constructed at LAD using a Valcor Model SV 515 metering pump as

the water injector. Distilled H^O was injected at a nominal

218
rate of 5-6 microliters/pulse onto a heated aluminum block

capable of flash vaporizing the water. The vapor was then mixed

with the flowing in a 20-gallon mixing chamber and carried

to the sealing station. The pulse rate of the metering pump was

controlled by a solid state timer and could be varied between 3

and 120 pulses/minute. The thermally large aluminum block was

heated by two cartridge heaters and regulated by a thermocouple/temp, controller. In all cases, the block

temperature was maintained at 150°C.

Using a 25m£ Buret and a separatory funnel as a reservoir,

the solid state timer could be calibrated by measuring the

decrease in Buret water level versus time. The humidifier was

capable of injecting up to 1000 microliters/minute into an

purge. The complete humidifier is shown in Figure 8.

2. The Sealing Station

The two IC sealing stations were located at NSC. One

station is designed specifically for sealing TO-3 packages while

the other can accommodate TO-5, TO-18, and T0-46’ IC packages.

Using the glovebox mounted on each station, the operator could

passes the samples into the sealing chamber via a "Bake-Out" oven

or small purge chamber, both located at the ends of the main

sealing chamber. During normal operation, dry flows through

the box at a nominal rate of 600 SCFH. During that portion of

the experiment where moisture was being added, the flow was

reduced to 100 SCFH through the humidifier.

There are two inlets to the sealing chamber from the

humidifier. Inside the chamber, the moisture level can be

219
monitored with two sampling lines. One line is held fixed near

the welding area while the other can be placed at any location in

the box to monitor moisture level gradients that may exist. Each

line can be toggled on or off separately. In normal operation,

both sampling lines are placed on opposite sides of the welding

area to assume an average moisture concentration.

3. The Dew Point Hygrometers

Two EG & G Model 660 Dew Point Hygrometers were used to

determine the moisture content in the sealing chamber. At a known pressure, the dew point of a gas is related to the H^O

concentration. Table IV shows this relation. For example, if

the Dew Point Hygrometer reads -20°C at 760mm Hg then the ppmV

H^O is 1020. One Dew Point Hygrometer was used to monitor an inlet line into the sealing chamber—effectively, the moisture

level in the humidifier. The other Dew Point Hygrometer

monitored either or both of the sampling lines. A small air pump

drew the moist ^through each Dew Point Hygrometer at a rate

of 3 SCFH. Cooling water was provided for the Dew Point

Hygrometer sensors to relieve cooling requirements at low dew

points at a non-critical flow of 1/2 GPM. The Dew Point

Hygrometer T s could be read to the nearest tenth of a degree C.

Refer to Figure 9 for a schematic description of the experimental

set-up.

THE EXPERIMENT : As stated previously, two complete sealing stations were needed

to seal all the samples. The procedure described is applicable to

220
both stations. At the termination of sealing packages at all

concentrations on one station, the equipment was disconnected and

reassembled on the other. Five package types: TO-3, TO-5 (flow 80),

T0-5 (flow 85), TO-18, and TO-46 were sealed at four concentrations;

dry, 3000 ppm, 6000 ppm, and 10000 ppmV H^O target value.

Initially, all caps and IC headers were baked at 150°C for one hour

in the side purge oven to remove any trapped moisture. After baking,

the units were transferred directly into the sealing chamber and

allowed to cool. It was necessary to seal the packages at the driest

level first and then increase the moisture level progressively. For

dry ^sealing, the purge line bypassed the humidifier and was

connected directly to the sealing chamber and the flow set at 600

SCFH. The dew points were monitored until a stable condition

existed. Usually, the dew point reached a minimum around -48.0°C

(approximately 50 ppmV H^O). Welded packages were passed out

through the side purge chamber. The next target valve was 3000 ppmV

H^O. It was necessary to reconnect the purge line t-o the

humidifier and cap the fitting on the sealing chamber. The heater

block in the humidifier was turned on and allowed to warm up. The

purge flow was reduced to 100 SCFH. After ten minutes the metering

pump was turned on and adjusted to the approximate pumping rate. The

rate can be calculated as follows:

The metering pump timer is calibrated in microliters/minute.

We know the target concentration of H9 0 to be 3000 ppmV

@ 100 SCFH dry Z

Use the rate equation: r = FC X 4.7195 X

10"^

K

221
Where r = Rate, microliters/minute

F = Dilution Flow, SCFH

C = Concentration, ppmV

K = Molar Volume Constant @ 25° = 24.47/M.W.

M.W. H

2

0 = 18

so r(microliters/minute)=(100 SCFH)(3000 ppmV X 4.7195 X 10

^

24.47/18

= 104 microliters/minute

After setting the pump to this value, the inlet Dew Point

Hygrometer was monitored to attain the actual moisture level. At the

same time the sampling line Dew Point Hygrometer came to an equilibrium value. Small changes were made to the pulse timer until

the sealing chamber dew point reached its proper value. Equilibrium

may take up to an hour or more depending on how much adsorption takes

place. The inlet Dew Point Hygrometer could fluctuate by as much as

_+ 0.5°C due to large moisture fluctuations at each pump pulse. The

dew point in the sealing area stabilized to within + 0.1°C

throughout the sealing time. This corresponded to a moisture level

accurate to approximately Hh 1%. Table V depicts the target values

and actual data taken from the experiment.

At the end of one moisture concentration, the cycle was

repeated. The humidifier was turned up, fine-tuned, and allowed to

come to equilibrium before sealing began. In all, a total of 3000

packages were sealed: 150 of each type at each concentration.

RESULTS

Table VI summarizes the results we have measured on these

packages. Results for each package type are grouped by target

222
concentration (50 ppmV, etc.) for each laboratory [Laser Analytics

(LAD), National Semiconductor (NSC), and the Mass Spectroscopic

facility (MS)] for each laboratory the number, n, in parentheses

[i.e. (23)] represents the number of packages measured, the next

number is the average concentration value, CBAR, and the third number

in the group is the one standard deviation value, s.d., for that

particular measurement.

Figures 10-13 also summarize the results obtained for T0-46 cans,

T0-18 results are summarized in Figures 14-18, and T0-3 results are

summarized in Figures 19-21. Data are presented as normal Gaussian

distributions with one standard deviation representing the width in

order to visually sense the overlap. Results for one set of

concentrations are summarized in each graph.

CONCLUSIONS

Readings for 50 ppmV on all systems are suspect for several

reasons. First, the values as well as the standard deviations

determined decrease with increasing volume. Secondly, we do not have

a model to explain the results. We do know that we can generate very

reliable results in flowing gas systems. We have made infrared

measurements on other TDL systems in the ppmV level. Additional work

will be necessary to better understand what we see. This will

probably require looking into different experimental configurations

to determine the effect of injection volumes and surfaces as well as

some attempt to better understand how much water is actually sealed

in one of these packages under the conditions imposed by the sealing

technique.

223
In general all techniques give rather large standard deviations

in the TO-46 can results. These results suggest a S/N problem for

the TDL systems, which were designed primarily to accommodate larger

volume packages. The standard deviations for Table VI appear to

support a system standard deviation (whether due actually to the

system or to the package sealing process) which is smaller than the

deviations measured in Table I.

In comparing the results from Table I with those from Table VI,

it appears that the Table I packages were probably sealed with

moisture concentrations of approximately 1500-2500 ppmV for T0-3,

between 2000 and 3000 ppmV for T0-5, and something between 2000 and

6000 ppmV for the T0-46 cans. Most likely results suggest all

moisture levels were sealed at approximately 2000 to 2500 ppmV.

The data in Table VII generally support the results of Figure 6

for all other packages.

GENERAL CONCLUSIONS

Surface area/volume ratio and TDL/MS ratio define a unique linear

function for the TO series cans we checked over the range of moisture

concentrations we were able to measure, with the exception of T0-46

cans. This is a particularly important result since the packages we tested were not prepared in any special way for the first experiment,

and the second experiment results were supportive.

Additional research is required to better understand the physics

and chemistry of the measurement process and the results for small

packages, especially with low concentrations. At this point low

concentrations (below 3000 ppmV) probably cannot be reliably

performed with any method on small volume cans (similar to T0-46).

224
REFERENCES

1. J. A. Mucha and P. R. Bossard, "Measurements of Water Vapor in

Integrated Circuit Packages using an IR Diode Laser,” Moisture

Measurement and Control for Semiconductor Devices, NBS (November,

1980).

2. P. R. Bossard and J. A. Mucha, IEEE Proceedings of the 19th Annual

International Reliability Physics Symposium, p. 60. (1981).

3. J. A. Mucha, Appl. Spectroscopy. 3(5, 393 (1982).

225
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TABLE II

Results* of R.A.D.C. TO-18 Round Robin (December, 1981)

-MASS SPEC. AT 100°C- -INFRAREDTARGET RADC LAB 1 LAB 2 LAB 3 100°C 25°C

1000 2663 2800 1981 3067 5420 2639

3000 4535 5730 3317 8690 8630 3724

5200 5327 5670 5631 4453 10,250 4753

6000 6040 7800 5417 6676 12,257 5460

10,000 10,060 9900 8301 9170 14,220 8897

All values in ppm by volume moisture.

227
COMPARISON

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229
TABLE V

TARGET VALUE

EXPERIMENTAL

DEW POINT

(DEGREES CENTIGRADE) PPMV H

2

0 + A PPMV @ + 0.1°C

Dry -48.0°C 49.7 ^ 0.61 ppmV

3000 ppm -8.2 2962 ± 25

6000 ppm +0.35 6186 + 48

10000 ppm +7.6 10318 + 68
RESULTS

TABULATED

BY

LABORATORY,

PACKAGE

TYPE,

AND

TARGET

CONCENTRATION

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253
4 . 5 USING PANAMETRICS MINI-MOD MOISTURE SENSORS

FOR MOISTURE MEASUREMENTS INSIDE SEALED

MICROELECTRONIC PACKAGES

David A Pinsky

Raytheon Company Missile Systems Division

Bedford, Massachusetts 01730

(617) 274-4252

Abstract

The use of Panametrics MINI-MOD HT moisture sensor chips to measure moisture intrusion

into sealed microelectronics packages is discussed. Limitations of these sensors and proper methods

for their employment are defined.

Introduction

Moisture sensor chips provide an inviting technique for directly measuring the moisture level inside sealed packages. At first look, these devices seem capable of providing repeatable moisture data

with ease. In fact, these devices are sensitive to temperature excursions so that their calibration be- comes unstable. Experimenters who overlook this fact can be led to erroneous conclusions.

As recently as September 1986 (Ref. 1), a paper was published where experimenters mistook

sensor drift for actual moisture data.

The intention of this paper is to describe the conditions under which good data can be obtained by using these sensors. In this way, the concerns raised in the last few years about hermeticity

acceptance limits (Refs. 2-6) can be addressed directly by well designed experiments.

Background

A study was performed to evaluate hermeticity criteria for large microelectronic packages as part of an RADC funded contract to investigate the effects of VLSI/VHSIC technology of MIL-STD883 test procedures.

The technique used for this work was to directly measure moisture levels inside sealed packages, and correlate these data with helium fine leak results obtained by MIL-STD-883 methods. Moisture contributions from internal organic materials was to be addressed as a side issue.

Packages

Thirty packages of four different styles were used in this study. A single Panametrics MINIMOD HT moisture sensor chip was mounted in each. Chip-to-package interconnections were provided

by 1-MIL aluminum wire.

254
Die attach was achieved by use of gold-silicon eutectric, non-conductive epoxy, or polyimide

film.

Table I summarizes the information on the packages and die attach used.

TABLE I - TEST SAMPLES

S/N Style

Number

I/O

Internal

Volume Die Attach

l Metal Can 58 5.24 cc Gold-Silicon Eutectic

2 Metal Can 58 5.24 cc Gold-Silicon Eutectic

3 Metal Can 58 5.24 cc Non-Conductive Epoxy

4 Metal Can 58 5.24 cc Non-Conductive Epoxy

5 Ceramic Leaded CC 132 0.26 cc Polyimide

6 Ceramic Leaded CC 132 0.26 cc Polyimide

7 Ceramic Leaded CC 132 0.26 cc Non-Conductive Epoxy

8 Ceramic Leaded CC 132 0.26 cc Non-Conductive Epoxy

9 Ceramic Leaded CC 132 0.26 cc Gold-Silicon Eutectic

10 Ceramic Leaded CC 132 0.26 cc Gold-Silicon Eutectic |

11 Ceramic Pad Grid Array 180 0.31 cc Polyimide

12 Ceramic Pad Grid Array 180 0.31 cc Polyimide

13 Ceramic Pad Grid Array 180 0.31 cc Non-Conductive Epoxy

14 Ceramic Pad Grid Array 180 0.31 cc Non-Conductive Epoxy

' 15 Ceramic Pad Grid Array 180 0.31 cc Gold-Silicon Eutectic 16 Ceramic Pad Grid Array 180 0.31 cc Gold-Silicon Eutectic

17 Ceramic Lcadless Chip Carrier 80 0.23 cc Gold-Silicon Eutectic 18 Ceramic Leadless Chip Carrier 80 0.23 cc Gold-Silicon Eutectic 19 Ceramic Leadless Chip Carrier 80 0.23 cc Gold-Silicon Eutectic 20 Ceramic Leadless Chip Carrier 80 0.23 cc Gold-Silicon Eutectic 21 Ceramic Leadless Chip Carrier 80 0.23 cc Gold-Silicon Eutectic 22 Ceramic Leadless Chip Carrier 80 0.23 cc Gold-Silicon Eutectic 23 Ceramic Leadless Chip Carrier 80 0.23 cc Gold-Silicon Eutectic 24 Ceramic Leadless Chip Carrier 80 0.23 cc Gold-Silicon Eutectic 25 Ceramic Leadless Chip Carrier 80 0.23 cc Gold-Silicon Eutectic 26 Ceramic Leadless Chip Carrier 80 0.23 cc Gold-Silicon Eutectic 27 Ceramic Leadless Chip Carrier 80 Q.23 cc Gold-Silicon Eutectic 28 Ceramic Leadless Chip Carrier 80 0.23 cc Gold-Silicon Eutectic 29 Ceramic Leadless Chip Carrier 80 0.23 cc Gold-Silicon Eutectic 30 Ceramic Leadless Chip Carrier 80 0.23 cc Gold-Silicon Eutectic

255
Initial Calibration

Each of the thirty packages had a pair of wires soldered to the external I/O leads to provide

access to the sensor signals. The packages were then placed in a dry-box with their leads passing

through a formed-in-place silicone gasket.

Figure 1 is a schematic diagram showing the apparatus used for calibration (and subsequent

testing). A Panametrics System I Hygrometer System was used to monitor the effluent from the dry

box. The moisture level was controlled by adjusting the rate of dry nitrogen flow through the system.

Output signals from the sensors were fed into a panametrics model 771 hygrometer which is specially designed for this purpose. The voltage from the model 771 was read out on a separate digital

voltmeter.

Calibration data was collected on each sensor. The dry box moisture level was varied from

4500 ppmv to 25 ppmv , as determined by the effluent.

Figure 2 is a plot of the calibration data taken from a single sensor over a period of several

days. The dry-box moisture level was slowly raised and lowered from about 20 ppmv up 500 ppmv

. No hysteresis effect was observed. These data indicated that the sensors can provide repeatable output.

Two data points shared the same voltage only when the dry box readings were different by no more

than 50 ppmv

. Using this type of data a calibration slope was assigned to each sensor. These slopes are listed

in Table II. At this point in the study, expectations were high that very small changes in internal moisture

levels could be measured over long periods of time.

Post-Seal Drift

Following calibration, every package was sealed by use of tin-lead solder preforms. (Sealing was performed in a dry nitrogen atmosphere held at about 50 ppmv of moisture.) The sealed packages were then returned to the calibration dry box for testing. The moisture level was raised and lowered

over a few days and the sensor outputs were recorded. Most of the sensors did not respond to changes

in the dry box environment, indicating hermetic-like behavior. Nine sensors responded dramatically

to the dry-box environmental changes, indicating leakage. These packages will be discussed in a later section.

The next part of the moisture intrusion study called for the packages to be monitored at room

ambient conditions. Data obtained during the first weeks after sealing showed the output voltages

steadily falling for all sensors. Quick calculations proved that this could not be due to an actual drop

in moisture content unless negative moisture levels were assumed.

Data was taken on all the sensors for three months following seal. Figures 3 and 4 show sensor output voltage data for two packages. Both voltages exhibit an exponential decay to a new, lower val256
Figure 1 - Schematic Diagram of the Apparatus Used for Initial Sensor Calibration and Subsequent

Testing

257
0.2350-

0.2300 -

MOISTURE LEVEL DECREASING 4/17

MOISTURE LEVEL INCREASING 4/18

MOISTURE LEVEL DECREASING 4/19

MOISTURE LEVEL INCREASING 4/22 AND 4/23

0.2250 -

SYSTEM 771

VOLTAGE

OUTPUT (V)

0.2200 -

0.2150-

0.2100

0.2050

0.2000 T" 150

l r~ 200 250

i 1 1 r~ 300 350 400 450

PARAMETERS SYSTEM 1 HYBROMETER READING (PPM)

Figure 2 - Initial Calibration Response of Sensor S/N 25. Data was collected with slowly increasing

and decreasing moisture levels on five different days.

258
TABLE II - INITIAL CALIBRATION SLOPES FOR ALL PACKAGES. SLOPES ARE FOR THE

RANGE 100-500 ppmv.

S/N Calibration Slope in ppmv/mV

l 5.8 2 5.8 3 No data - dead sensor 4 6.9

5 . 17.2 6 8.7 7 20.8 8 7.4 9 16.9 10 28.6

11 5.0 12 6.6 13 6.5 14 7.5 15 No data - dead sensor 16 22.7 17 21.4 18 30.3 19 24.7 20 26.3 21 28.6 22 31.3 23 53.7 24 25.3 25 26.3 26 20.8 27 22.4 28 27.0 29 24.3 30 26.6

259
uc. Figure 5 is the data from Figure 3 on a semi-logaryhmic plot. This slope indicates a time constant

of about 33 days.

This type of drift data was collected on all sensors and is summarized in Table III. Some of

the sensors did not achieve a stable output during the entire three month period.

TABLE III - DATA ON DRIFT FOR OPERABLE SENSORS MOUNTED IN APPARENTLY

HERMETIC PACKAGES FOLLOWING PACKAGE SEALING

S/N

DC Voltage

At Stabilization

Total Amount of

Negative Drift As

a % of Final Value

Approximate

Time Constant

For Drift Decay

In Days

1 .2469 1.9 20

3 .2655 0.6 20

4 .3125 1.6 34

5 .2246 3.1 78

17 .2050 3.5 32

18 .2056 7.1 30

19 .2046 5.7 32

20 .2146 6.1 34

21 .2835 13.7 34

22 .2590 17.7 32

24 .2150 4.9 35

25* .3138* 29.6* >133*

26 .2099 4.4 33

27 .2500 12.3 34

28 .2160 6.2 32 29 .2134 3.3 32 30 .2030 6.0 37

* S/N 25 never stabilized. Data given is for 200 days after seal.

Another form of sensor failure also occurred during this investigation when some sensors suddenly provided very high or very low output voltages and would no longer respond. These failures may

have been due to internal short or open circuits, or other problems.

Moisture Intrusion - Data

Despite these calibration difficulties, some interesting moisture intrusion data was obtained.

For the purposes of obtaining qualitative results, the initial calibration data was used to convert volt- ages into apparent moisture values. These values can be used only to compare the relative moisture

levels inside the packages used in this study.

260
AVERAGE OUTPUT

VOLTAGE FOR NINE

APARENTLY HERMETIC

_!

Figure 3 - Average Voltage from Nine Sensors with Apparently Hermetic Behavior Showing Post-Seal

Drift which Assympototically Approaches the Value 0.2086

261
0.400

0.3900-

0.3800 < a

0.3700-

OUTPUT

VOLTAGE 0.3600

0.3500

A

A 0.3400 -

0.3300

0.3200

0.3100 t 25 50 75 100 125 150 175

TIME IN DAYS

\

\ \

Figure 4 - Voltage from the Sensor in an Apparently Hermitic Package Showing Post-Seal Drift.

Stabilization has not occurred within 175 days.

262
Figure 5 - Post-Seal Drift Data from Figure 3 Fit to the Curve: V = Va + (V, - VJe^F with

VQ = 0.2086 and V, = 0.2207

263
Figure 6 is a plot showing the apparent moisture levels in two packages which were cycled between room ambient and about 50 ppmv in the dry box. These data clearly indicate the immediate effect of the dry box moisture on the apparent package moisture. Hereafter packages displaying this

form of behavior will be referred to as “apparent gross leakers”. Most of the other sealed packages

showed no response to the dry box moisture level and therefore will be described as “apparently hermetic”.

Figure 7 is a plot of the apparent moisture content in three packages during the three months

at room ambient. Package S/N 28 is apparently hermetic. The sensor drift can be seen as a small drop

in the apparent moisture content. Package S/N 8 is an apparent gross leaker. Its apparent moisture

content immediately rises to between 2000 to 2500 ppmv and remains there. Package S/N 7 exhibits a quite different behavior. Its apparent moisture level rises and asympototically approaches the apparent moisture level of package S/N 8. About seventy days are required for the moisture rise to be complete. This package will be referred to as the “borderline leaker”, as it is clearly in the region between

gross leakage and hermetic behavior.

At the end of three months at room ambient the remaining stabilized, apparently hermetic

packages were divided into two groups. One group was maintained at room ambient. The other group was placed into a chamber at 95 percent RH. (Previous plans for temperature cycling had to be discarded.)

The apparent moisture content in most of these packages showed no signs of increase or decrease during this time. Figure 8 is a plot of the changes in the apparent moisture content of one package which was exposed to 95 percent RH (wet) and one which was exposed to room ambient (dry). Table IV provides the number of data points taken and their standard deviation in terms of apparent

moisture content for all packages of both groups (except those discussed below).

Two packages held at high humidity did display large increases in moisture content. Both were

metal packages which experienced corrosion at the seal. Figure 9 is a plot of the apparent moisture

content in one such package showing two sharp increases in the moisture intrusion rate. This behavior

is consistent with the effects of corrosion.

An apparent moisture intrusion rate was assigned to each package with a functioning sensor. Assignments for the apparently hermetic packages were made by assuming that the increase in apparent moisture level were less than 25 ppmv , which is approximately equal to the standard deviation of

their moisture level data. Table V summarizes these rates.

Moisture Intrusion versus Helium Leak Rates

A goal of this study was to correlate moisture intrusion rates with helium leak rates. To this

end all but one of the packages were helium leak tested at the end of this study.

Gross leak testing was performed on all packages (except no. 2 which died early) per MILSTD-883C, Method 1014, Test Condition C|, Fluorocarbon Gross Leak, fixed. Parts were bombed at 60 psig for 2 hours.

264
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265

Figure 6-

Response

of

Two

Sensors

To

Changes

in

Dry

Box

Conditions.

Apparent

Gross

Leakage

is

Indicated
APPARENT

CHANGE

IN

PACKAGE

MOSITURE

PPMV

266

Figure 7-

Post-Seal

Response

of

Three

Sensors

During

87

Days

at

Room

Ambient.

Package

S/N 8

is

an

apparent

gross

leaker.

Package

S/N

28

is

apparently

hermetic.

Package

S/N 7

is

the

borderline

leaker.
>

20 40 100

TIME (DAYS)

S/N 29 DRY

• S/N 24 WET

Figure 8 - Post Stabilization Behavior of Two Sensors in Apparently Hermetic Packages. One package

S/N 29, was held at room ambient, while S/N 24 was exposed to 95 percent relative humidity.

267
TABLE IV - APPARENT MOISTURE INTRUSION DATA FOR HERMETIC PACKAGES AFTER

STABILIZATION

S/N

External

Environment Data Points

Standard Deviation

in ppmv

3 Dry 13 4

17 Wet 8 11.3

18 Wet 11 17.9

19 Wet 11 28.5

20 Wet 11 24

24 Wet 11 18.8 26 Dry 14 20.5

27 Dry 13 11.1

29 Dry 14 18 30 Wet 8 22.7

Eighteen packages which passed gross leak test were then tested per MIL-STD-883C, Method

1014, Test Condition A1} Helium Fine Leak, fixed. A bomb pressure of 60 psig was used and the part was baked for 10 minutes at 100°C to accelerate the 1 hour allowed waiting period. Leak rates were measured and recorded for all packages.

To perform an analysis, all moisture intrusion data was converted to units of g/s intrusion of

water. This absolute measure of moisture intrusion should relate directly to absolute helium leak rate expressed in atm-cc/s. All of the packages in this study were sealed with solder preforms, thus the ef- fects of package geometry should not be significant. Though seal areas and package volumes may vary, any existing leakage path would exist for both helium and water. Thus, it is reasonable to expect a correlation between absolute helium leak rates and absolute moisture intrusion rates. Table VI summarizes all of the helium leak rate data and the apparent moisture intrusion data

converted into absolute units. Figure 1 0 is a plot of log10 of the apparent moisture intrusion rate versus

log10 of the measured helium leak rate. Every gross leakers was assigned a helium leak rate of > 1 O'5 atm-cc/s because this is the upper limit of measurement on our equipment. Two vertical lines are drawn on Figure 10. The right hand line corresponds to a helium leak rate of 5 x 10 8 atm-cc/s and

the left hand line to 1 x 10 8 atm-cc/s. The data point corresponding to package S/N 7 falls between

these lines. This package is, therefore, considered to be hermetic by MIL-STD-883. However, moisture

intrusion data shows that this package achieved ambient levels of moisture in just 70 days at room ambient, which is cause for some concern.

Sensor Performance - Discussion

The sensor instabilities resulting from a temperature excursion poses a serious difficulty for the

experimenter. It is clear from the data that the calibration curves change not only in DC voltage offset,

but also in slope. Thus, moisture changes cannot be determined absolutely.

268
Figure 9 - Apparent Moisture Intrusion into S/N 1 (Induced Leaker) During Exposure to High

Humidity. Arrows indicate times at which leak rate appears to increase. Moisture levels are for

. comparison only. Absolute levels could not be determined due to sensor calibration shift.

269
TABLE V - POST-STABILIZATION ENVIRONMENT, APPARENT MOISTURE INTRUSION

RATE OR FAILED SENSOR STATUS, AND HERMETICITY DESIGNATION FOR EACH OF THE

THIRTY PACKAGES

S/N

Wet or Dry

Environment

Moisture Intrusion

Rate (ppmv/S)

Apparent

Hermeticity

l Wet >5.4 x 10*3 Induced Leaker

2 - No Data - Sensor Died —

3 Dry <3 x 10 6 Hermetic

4 Wet >1.5 x lfr2 Induced Leaker

5 - No Data - Unstable Sensor —

6 - No Data - Unstable Sensor —

7 Dry 4.6 x 10^ Borderline Leaker

8 Dry >6 x 10 1 Gross Leaker

9 Dry >6 x 10 1 Gross Leaker

10 Dry >6 x 10 1 Gross Leaker

11 Dry >6 x 10 1 Gross Leaker

12 Dry >6 x 10 1 Gross Leaker

13 Dry >6 x 10 1 Gross Leaker

14 Dry >6 x 10 1 Gross Leaker

15 - No Data - Sensor Died —

16 Dry >6 x 10 1 Gross Leaker

17 Dry <3 x 10 6 Hermetic

18 Dry <3 x 10 6 Hermetic

19 Dry <3 x 10 6 Hermetic

20 Dry <3 x 10 6 Hermetic

21 - No Data - Unstable Sensor —

22 - No Data - Unstable Sensor —

23 - No Data - Sensor Died —

24 Wet <3 x 1- 6 Hermetic

25 - No Data - Unstable Sensor —

26 Dry <3 x 1- 6 Hermetic

27 Dry <3 x 1- 6 Hermetic

28 - No Data - Sensor Died —

29 Dry <3 x 1- 6 Hermetic

30 Wet <3 x 1- 6 Hermetic

270
TABLE VI - APPARENT MOISTURE INTRUSION RATES CONVERTED TO UNITS OF g/S,

AND MEASURED HELIUM LEAK RATES FOR ALL PACKAGES

S/N

Moisture Intrusion

Rate (g/s)

Helium Leak

Rate (atm-cc/s)

l >2.3 X 10 11 >10 5

2 No Data No Data

3 <1.3 x 1- 14 6.2 x 10 9

4 >6.3 x 10 11 >10 5

5 No Data 2.7 x 10-9

6 No Data 1.5 x 10 9

7 9.6 x 10 14 3.5 x 10^

8 1 >1.3 x 10 10 >10 5

9 >1.3 x 10 10 >10 5

10 >1.3 x 10 10 >10 5

!

11 >1.5 x 10 10 >10 5

12 >1.5 x 10 10 >10 5

13 >1.5 x 10 10 >10 5

14 >1.5 x 10 10 >10 5

15 No Data >10 5

16 >1.5 x 10 10 >10 5

17 <5.6 x 10 16 1.6 x 10 9

18 <5.6 x 10 16 1.6 x 10 9

19 <5.6 x 10 16 1.6 x 10 9

20 <5.6 x 10 16 1.6 x 10 8

21 No Data 1.9 x 10 9

22 No Data 6.5 x 10 9

23 No Data >10 5

24 <5.6 x 10 16 0.8 x 10 9

25 No Data 1.3 x 10 9

26 <5.6 x 10 16 1.5 x 10 9

27 <5.6 x 10 16 0.7 x 10 9

28 No Data 0.7 x 10 9

29 <5.6 x 10 16

j 0.7 x 10 9

30 <5.6 x 10 16 1.1 x 10 9

271
- 9=

- 10 -

© 84 I/O LCC

h 132 I/O LDCC

182 I/O PGA

v 58 I/O CAN

- 11 -

log 10 (MOISTURE

INTRUSION RATE IN g/sec)

-13-

- 14-

15- Ml

-10

T

-9

log 10 (HELIUM LEAK RATE CC-ATM/sec)

-5

Figure 10 - Correlation of Apparent Moisture Intrusion Rates with Measured Helium Leak Rates. Small arrows indicate region bounded by each datum. Numerals indicate a number of data falling on

the same location.

272

rr
However, some information about the calibration shifts can be inferred from the existing data.

The apparent moisture level corresponding to room ambient conditions always came out as about

2500 ppmv

. If room ambient is taken to be 40 percent RH at 20°C (which approximates our air conditioned lab in summertime), the actual moisture level was around 14,000 ppmv

. This implies that the

new calibration slopes should be approximately 5.6 times their original values.

For the purposes of the analysis given in Figure 10, such a relative slope change is not very

significant. Determination of actual leakage into apparently hermetic packages is, however, impossible.

Another factor affecting the sensor performance was the nature of the interconnections. Slight

differences in output voltages (a few tenths of a millivolt) resulted from moving the location along the

tinned-leads where the clips were attached from the model 771 hygrometer. Half a centimeter was

enough of a movement to show in the output. All readings were taken with clips attached at the edge

of the insulation to aid repeatability.

Sensor Utilization - Recommendations

The primary concern of the experimenter using these sensors is to avoid variations in temperature. Temperature excursions result in calibrations that change with time. Thus, the entire experimental sequence should be as isothermal as possible. Calibration should be performed at the end of the

sequence.

The mounting of dice and sealing of packages almost always entails the use of elevated temperatures and this in turn results in unstable sensor responses that may take several months to stabilize. During this period of instability, precise measurements cannot be made - only changes of thousands

of ppmv can be inferred during this time.

One concept to get around this difficulty is to enclose sealed packages with mounted sensors

in a dry box held at a constant, low moisture level equivalent to the seal atmosphere. This dry box

should be field isothermally. The sensor outputs should be monitored until adequate stability is achieved. Once stability is reached the packages can be exposed to higher levels of external moisture

(isothermally!). Data could then be obtained with leaking packages starting from a known moisture level.

This technique alone will not help to define the moisture contribution of organics sealed in a package. Outgassing that occurs within the first few months of sealing cannot be measured directly.

Only through statistical methods can these sensors be used to determine moisture evolving inside a package.

The difficulties encountered with dying sensors could be avoided by mounting multiple sensors

in a package. This could also help to reduce uncertainties associated with internal moisture determination. However, the experimenter must trade off the increased cost of additional sensors (about $50

each) against the use of additional packages.

Providing stable interconnections from the packages to the sensor bridge is important as sol- dering could result in sensor instability. The use of sockets are recommended.

273
One of the difficulties in performing experiments to determine optimum accept/reject criteria

is the dearth of marginal packages. Both hermetic and gross leaking packages are fairly easily obtained.

The marginal or borderline leakers are rarer. Out of thirty seals performed in this study only one was

truly marginal, yet an entire distribution of marginal packages is required. The only way to insure that

many marginally sealed packages are made is to seal large numbers of packages. The use of at least a few hundred packages will be needed if more than a few marginal leakers are desired.

Conclusions

Panametrics MINI-MOD HT moisture sensors have many crucial limitations which must be

accounted for if optimum quantitative data is to be obtained. Qualitative data and relative quantitative data can be fairly easily obtained using these sensors.

Recommendations

Detailed recommendations are discussed above, but a short list of do’s and don’ts for experimenters using these sensors follows:

Avoid all temperature excursions.

Allow sensor output to stabilize after any temperature excursions. This may require several months.

Calibrate sensors after stabilization.

Use sockets or other such devices to provide repeatable interconnections between packages and the sensor bridge.

Use enough sensors to provide reasonable statistics. Assume that 10-20 percent of all sensors will fail.

274
REFERENCES

1. A.D. Barany, et. al., “Hybrid Microcircuit Moisture Measurement Using Moisture Sensor Chips”, Hybrid Circuit Technology, Sept. 1986.

2. D.A. Pinsky, “Hermeticity Measurements For Large Microelectronics Packages”, Proc. of the Inti. Symp. on Microelectronics, ISHM 1986.

3. D.W. Swanson and J.J. Licari, “Effect Of Screen Tests And Bum-In On Moisture Content Of Hybrid Microcircuits”, Sblid State Technology, Sept. 1986.

4. David, R.F.S., “Testing Hybrids For Water Content”, Test and Measurement World,

Feb. 1986.

5. A. Dermarderosian and V. Gionet, “Water Vapor Penetration Into Enclosures With

Known Air Leak Rates”, IEEE Trans, on Electron Devices, Jan. 1979.

6. D. Stroehle, “On The Penetration Of Gases And Water Vapour Into Packages With

Cavities And On The Maximum Allowable Leak Rates”, IEEE Rel. Phys. Symp. Proc.,

1977.

275
4 . 6 : Aluminum Oxide Sensor: A Tutorial Update

V. Fong

Panametrics

PAPER NOT AVAILABLE

276
5.1 RELIABLE PLASTIC ICs FOR AUTOMOTIVE ELECTRONICS

Robert J. Straub

Ass’t Staff Engineer, Parts Engineering

Delco Electronics Corporation

Mail Station R117

Kokomo, IN 46902

Phone (317) 451-7027

ABSTRACT

At Delco Electronics Corporation, we use almost one and one half million

plastic-encapsulated integrated circuits per day. Many of the systems that use

these ICs have a five-year or 50,000-mile warranty. To obtain this mandated

high reliability required an effective parts program, complete specifications,

adequate testing and a partnership with our key suppliers. This paper

discusses some of the requirements to obtain these reliable integrated

circuits. Some data is presented on what the warranty costs could be if a small percentage of parts would have to be replaced. Charts of plastic IC

failure mechanisms observed during calendar years 1980 and 1985-86 are shown

along with information showing significant improvements in reliability. Our

suppliers have proven that we can work together to obtain the needed

reliability utilizing plastic-encapsulated integrated circuits.

A CHALLENGE

In 1977, when Delco Electronics was given the charter to develop engine control

computers for General Motors automobiles, a need for high-quality, reliable,

low-cost integrated circuits (ICs) became a necessity. These engine controls

must be reliable to support a minimum of a five-year or 50,000-mile warranty.

For example, if one integrated circuit had a usage of one per automobile and, over a five-year period, there were 20 million automobiles made using this

electronics system and 0.1% (1000 PPM) of this part had to be replaced, the

warranty, costs could be $2,600,000 because of a single, unreliable IC. This

presumes there were no other failures.

At Delco Electronics, we use almost one and one half million plasticencapsulated integrated circuits a day. That’s an estimated 354 million for

our 1987 Model Year. Thirty-eight (38) million of these are surface-mounted

devices. We had to find ways to assure high quality and reliable ICs or customer dissatisfaction and warranty costs would severely damage our reputation and future business growth.

277
INVESTIGATION

During 1977 and 1978, we began a thorough investigation to find adequate

integrated circuits to meet our goal of high-quality, reliable parts at

cost-effective prices.

Military/Aerospace Parts

We reviewed the use of MIL-M-38510/MIL-STD-883 integrated circuits. They were

not only too costly, but not enough capacity was available in the world to

supply Delco Electronics* projected needs. Ceramic packages are also very

fragile and, therefore, hard to handle in automatic assembly machines.

Commercial/Consumer Electronics Parts

Commercial plastic ICs were low cost, but the semiconductor industry did not

provide specifications over the automotive temperature range of -40 to 85 or

125°C. These ICs had inadequate testing, even at room temperature. The

thermal resistances of 24 and 40-pin plastic dual-in-line-packages (DIPs) were

too high for use with complex high-power dissipation ICs in the high

temperatures. Some manufacturers' ICs were unreliable in a humid environment,

others would fail during the cycling of the ambient temperatures in the

automotive environment; however, some manufacturers' ICs were good quality and

were reliable in the automotive environment.

WHAT WE DID

Delco 's approach to obtaining reliable plastic ICs was to have an effective

parts program. It was based on previous experience in the automotive

environment (primarily for radios, voltage regulators and ignitions) and

knowledge obtained in the specification and use of integrated circuits in

military and aerospace applications. This background allowed us to take a fresh approach to procuring extremely reliable ICs at cost-effective prices

that would meet the demands of the automotive environment and the five-year or 50,000-mile warranty. This program began in 1977 with the transfer of some

engineers from Delco 's Santa Barbara, California Operation, where they had

worked on military and aerospace ICs, to Kokomo, Indiana and the utilization of

experienced automotive engineers from Kokomo.

Complete Specifications

Part Drawings

The IC part drawings were modeled after the military/aerospace drawings used in

Santa Barbara, except they did not reference any military specifications

because of the fear that if a military specification was used, our suppliers

would feel that was an invitation to increase prices substantially.

278
The present IC part drawings contain:

Mechanical Requirements - Mechanical requirements describe the plastic package outline

and the thermal resistance junction to ambient as it is

mounted on a printed circuit board in still air. We often

cannot allow Alloy 42 lead frames because they do not

achieve a low enough thermal resistance which is needed to

meet the absolute maximum junction temperature restriction

of 150°C for the power dissipations of the particular IC.

Often copper lead frames are used to achieve the lower

thermal resistance required.

Maximum Ratings - Storage Temperature Range

Typically -50 to +150°C.

Operating Ambient Temperature Range

Typically -40 to either 85 or 125°C

Maximum Voltages and Currents on Pins

Absolute Maximum Junction Temperature

Never over 150°C for today’s plastics.

Electrical Performance Requirements - An outline of the voltages, currents and dynamic

requirements that must be designed into the IC with process

controls to assure they are met regularly.

Electrical Test Requirements - DC parametric

AC (Switching times)

Functional

Adequate stuck-at-one and zero fault coverage of digital

circuits

Each of these electrical test requirements are specified at -40, 25 and a high

ambient temperature (85 or 125 °C) depending on if the system is mounted in the

passenger compartment or under the hood.

High Test Temperature - Based on the thermal resistance and the maximum specified

power dissipation as the IC is used in the application.

The high test temperature is usually specified at a higher temperature than the

maximum ambient operating temperature because the industry practice has been to

test the ICs on automatic testers which test the parts in a very short time.

This short time does not allow the IC die to reach thermal equilibrium and,

therefore, does not assure that the part will work at steady-state conditions

in the maximum ambient in the real world. An example is an IC that dissipates

400 milliwatts housed in a package that has a thermal resistance of 100°C per

watt as soldered into the system circuit board which must operate in an ambient

temperature up to 85 °C.

279
The steady-state junction (die) temperature will be:

Tj = Ta + Pd*theta ja

where Tj - Junction temperature.

Ta - Ambient temperature within the system.

Pd - Power dissipation within the IC.

theta ja - Thermal resistance from junction to ambient as the IC is

mounted in the application.

Tj = 85 + .400 * 100

= 125°C

The actual steady-state junction temperature in an 85°C ambient is 125°C.

When tested on high-speed test equipment, the junction never reaches 125 °C and,

therefore, we are not assured the IC will function properly at the maximum

operating ambient temperature.

The method of specifying the high test temperature utilizes an assumption that

the test time is zero. It is obvious that a zero test time is impossible.

This method provides a guardband. This is the high test temperature the

handler or test chamber must be set at to measure the 85 °C tests. This method

onxy works where the test times are very short. The IC die does begin to warm

up while being tested, but it never reaches thermal equilibrium if the test is

quick. This high test temperature would not work for manual test equipment

where test times are long. The time to reach thermal equilibrium for dual

in-line packages is usually more than three minutes, but the rate of change is an exponential and quite complex, depending on the package thermal time

constant, the method of heating the part, etc.

Lot Acceptance -

Delco requires every IC wafer lot to pass acceptance tests of autoclave and

burn-in prior to being shipped for use in production. Our suppliers must do

this lot acceptance at their facilities.

Autoclave

48 hours at 121°C, 15 psig.

with no defectives allowed,

sampled.

This is a test which replaces the test on hermetic ICs where 100% fine and

gross seal screening is performed. There is no screen to eliminate ICs molded

improperly. Autoclave is considered a destructive test. If the sample fails

autoclave, the entire lot must be scrapped. The IC wafer fabrication, molding

process and materials must be under good statistical process control to

minimize losses.

The sample plan is 22 samples

Every wafer lot must be

280
Burn-in

46 hours at 125°C with bias applied.

Bias must be retained until the IC is cooled down. No high-temperature exposure

is allowed between the time of bias removal and the endpoint testing at room

temperature. The endpoint test must be made within 48 hours of the removal of

bias. Burn-in lot acceptance evolved as follows:

100% burn-in was required on engine control ICs in 1979 while the

volumes were low.

This gave us time to gather data which supported that a lot acceptance gate

would be an effective means of preventing defective lots from being shipped to

Delco. Refer to Figure 1 which shows the evolution of the burn-in lot

acceptance sampling plans operating characteristic curves from before 1977

through 1986.

Electrostatic Discharge (ESD) Sensitive - A Warning to treat all integrated circuits as Electrostatic

Discharge Sensitive. Integrated circuits which were

degraded at less than 2000 volts are treated as extremely

ESD sensitive.

Common IC Specification - A Reference to the Common IC Specification which contains

added requirements.

FIGURE I BURN-IN LOT ACCEPTANCE SAMPLING PU\N

OPERATING CHARACTERISTIC CURVES EVOLUTION

1977 1986

PROBABILITY OF ACCEPTANCE

281
Integrated Circuit Specification

This integrated circuit specification is applicable to all integrated circuits.

This specification defines the requirements common to all plastic ICs and also

calls out the qualification specification which defines specific environmental

test conditions. These specifications contain accelerated tests to assure the

IC will operate reliably during the life of the vehicle.

Automotive and/or Manufacturing Environments - Temperature Cycle

1000 cycles -40 to either 125°C or 150°C*

Power and Temperature Cycling

500 cycles with bias from -40 to either 85°C or 125°C*

High Temperature Operating Life

1000 hours with bias at 85°C or 125°C*

* Passenger compartment application uses lower temperature. Underhood uses

higher temperature.

Automotive and/or Manufacturing Environments - (Continued)

Autoclave

48 hours, 121°C, 15 PSIG

Bias Humidity

1000 hours, cycled 30 to 65°C, 90 to 98% RH

High-Temperature Storage Life

1000 hours at 150°C

Solderability

Solder Heat

Lead Bend/Pull

Thermal Resistance

Board Wash Test

Solvent Resistance

Test Grading

Test Grading/Fault Grading is a requirement that all digital ICs must have

their test pattern "graded” to assure it will detect at least 99% of all the

possible stuck-at-one and zero faults on every input and output of every gate

on a gate-level model of the IC. We have found this grading necessary to

assure our test patterns would catch defective ICs. This requirement has

forced our suppliers to design in testability.

Test Software Control -

The IC test software is audited to assure the tests are actually performed as they are called out in the part drawing. The software, including the test

graded patterns, are under Engineering Change Order control.

282
Electrostatic Discharge Capability Requirement -

a) 2000 volts, 100 pf, 1.5k ohm and

b) 200 volts, 200 pf, no resistor

Shipping Containers -

Requirement that all ICs must be shipped in antistat-coated conductive shipping

tubes or conductive tape on reel. All ICs are treated as ESD sensitive within

Delco.

Today, an average of 550-700 samples are used to qualify a surface-mounted

integrated circuit. Refer to Figure 2 to observe the evolution of the

operating characteristic curves for the sampling plans for each environmental

qualification test. Each environmental test is usually run in parallel (i.e.,

temperature cycling on a single group of 77 ICs, operating life on another

group of 77, etc.)

FIGURE 2

QUALIFICATION SAMPLING PLAN

OPERATING CHARACTERISTIC CURVES EVOLUTION

1983 1986

N=31 A=1 N=77 A=0

PERCENT DEFECTIVE

283
Changes -

We specify that no significant processes, design, or manufacturing wafer or assembly site changes are allowed

without prior approval from Delco. When there are significant changes in design, processes, material, or manufacturing site which may degrade the reliability or impact the application of the IC, we may require a requalification plus assembly into over 1000 systems to

assure compatibility and reliability prior to authorizing

the change.

Qualification Program

We performed all the applicable environmental, mechanical, life and electrical

transient tests at Delco until we were thoroughly familiar with the results on various types of parts. As we became experienced, we were able to do some qualification by similarity. We also have some of our key suppliers run qualification tests in parallel with Delco* s tests and, if the supplier

expressed a commitment, we correlated in our test results and, if the supplier

passed our on-site survey, it may be certified to do self-qualification under

Delco*s directions.

Rigid Receiving Inspection Testing

0.1% AQL in 1979; today, no defects allowed

Testing at -40°C, Room, and Hot

Solderability Test

Mechanical Dimensions

Some suppliers’ sites have been certified so that Delco no longer has to do

incoming inspection verification. These suppliers have made a management

commitment to be a partner with Delco, have had extremely successful results in

incoming inspection, have avoided infant failures on the manufacturing line and

have proven to have reliable parts from field experience. This program is

called NIV (No Inspection Verification) at Delco. Delco still monitors the

suppliers’ quality performance in other ways.

Partnership With Our Suppliers:

No Inspection Verification (NIV) . The no incoming inspection program

described above.

The Certification to do Self-Qualification Testing as described

earlier.

Failure Analysis performed by supplier on failures found at Delco.

Regular interface meetings between Delco Quality Control, Purchasing

and Parts Engineering and the suppliers’ Quality Control, Sales and

Engineering representatives.

284
At these supplier interface meetings, held alternately at Delco and at our suppliers', the delivery performance, qualification testing data, incoming

inspection results, line removal data, field performance, lot acceptance

results, failure analysis findings and any problems are reviewed. We work

together as teams to obtain optimum reliability.

Failure Analysis

Performed both at Delco and/or supplier.

Receiving Inspection failures are analyzed.

Line removals are studied.

System level burn-in and reliability monitor failures are analyzed

Assembly plant part failures are evaluated.

Warranty returns are analyzed.

We monitor these analyses for unacceptable trends and corrective action teams

identify causes and implement corrective action.

RESULTS : Qualification Testing

Over the last year, Dilco tested 68,622 parts to qualify 263 new devices. Our

suppliers tested 44,973 parts to qualify 95 new devices. In addition to

qualification tests, Delco tested 11,131 parts to evaluate 73 new ICs under

development

Lot Acceptance Summary

During the last year, Delco's suppliers tested more than 219,000 ICs in

autoclave at lot acceptance and they had an average fallout of 0.0407%

(407 PPM) . Those lots that failed the acceptance criteria were not shipped to

Delco. During this same time, over 1.7 million ICs were subjected to the

sample lot acceptance burn-in at our suppliers' with an average fallout of

.0830% (830 PPM). Those lots that failed the burn-in sampling acceptance

criteria had to be 100% screened at the supplier. These came from more than

10,500 lots from ten suppliers. Figure 3 shows the relative improvement over

the last few years.

28 5
FIGURE 3

LOT ACCEPTANCE AUTOCLAVE Sc BURN-IN RESULTS

FALLOUT IN PPM (INDUSTRY AVERAGE)

a

zze

£

CL

1084 1086

MODEL YEAR

C.Qulmby 7-18-86

Receiving Inspection Electrical Results

Figure 4 shows the relative improvement observed in the Receiving Inspection

electrical testing of the large scale integrated circuits (LSI) from Model Year

1980 to 1986.

286
Manufacturing Line Removal Results

Figure 5 exhibits the relative improvement observed in the reduction of

manufacturing line removals of complex LSI level ICs from Model Year 1979

through 1986.

Integrated Circuit Warranty Return Results

Figure 6 shows the relative improvement in either the observed or predicted,

depending on the age of the systems, 5-year /50, 000-mile warranty returns for

complex LSI integrated circuits.

287
NMOS Failure Mechanisms 1980 Versus 1985-86

Figures 7 and 8 show the failure mechanisms we found during failure analysis of

NMOS LSI manufactured in calendar year 1980 by two different suppliers. Note

that each, supplier has its own problems. Figures 9 and 10 show the failure

mechanisms we found during failure analysis of NMOS LSI manufactured in 1985-86

by the same two suppliers as in Figures 7 and 8. Note how the failure

mechanisms have evolved. Also keep in mind that the overall failure rates have

become much lower during this time span.

FIGURE 7 "BEFORE" NMOS FAILURE ANALYSIS RESULTS

VENDOR = "A" YEAR MANUFACTURED = l^BO

EOS 9.5%

PERCENT OF FAILMECH

FIGURE S "AFTER" NMOS FAILURE ANALYSIS RESULTS

VENDOR - "A" YEAR MANUFACTURED *= 19B5. 86

DIE UTL-CORROS 5.3% EOS 5.3%

OTHER 10. 5%

/ N. ESD/TRANSIENT 21.1%

/ UASK/ETCH DEE 12.3%

WIRE—BOND 45.6%^"^^

PERCENT OF FAILMECH

288
CONCLUSION:

Plastic-encapsulated integrated circuits are not inherently more or less

reliable than hermetic ICs. A complete parts control program must be utilized

to obtain highly reliable ICs. At Delco Electronics, we have proven that

plastic ICs can meet our stringent reliability requirements.

The integrated circuit industry has made tremendous strides in improving

reliability in the last few years. The reliability goal of Delco Electronics

and General Motors is to continue this work and drive toward zero returns.

Delco and our suppliers have strong programs to reach toward this goal.

289
5.2 A TEST PROGRAM TO ASSESS RELIABILITY OF PLASTIC PACKAGES AND GEL

COATINGS FOR MILITARY ENVIRONMENTS

EUGENE C. BLACKBURN

Rome Air Development Center

Griffiss AFB NY 13441-5700

And

DR. DIDIER KANE

Clarkson University Contractor

Rome Air Development Center

Griffiss AFB NY 13441-5700

I. INTRODUCTION

The military microcircuit user community has through years of experience

defined certain basic requirements that electronic devices must possess if they are to

perform satisfactorily in their functional environment. The codification of this user

experience is known as MIL-M-38510 (Specifications for Military Microcircuits) and

MIL-STD-883 (Test Methods for Military Microcircuits). One measure of the

appropriateness of these documents is the frequent citing of them by users in the

automotive, industrial, and severe environment commercial applications. The

requirements contained in these documents are based on the presumptions derived

through historical evidence, that only hermetically sealed packages can adequately

protect semiconductor devices. Over a period of many years, the Department of

(1) (2 3) Defense (DOD) and commercial organizations ’ have studied alternative

packaging choices, including injection molded plastic, premolded plastic cavity, chip

on board with conformal coating, combinations of organic and inorganic materials,

and other options too numerous to mention. Each of these approaches shares the

attribute that it replaces hermeticity with die protection designed to isolate the chip

from its environment. This protection retards failures by slowing the speed with

which corroding elements reach the die.

290
II. DOD INTEREST

Organic compounds are now available with higher levels of purity than were

ever offered before. These materials are very low in ionic constituents and by their

molecular design can limit the quantity of corrosion promoting materials that reach a

(it) silicon die. Test results have been reported by Hitachi and internal studies have

been conducted at AT&T

^

indicating that certain silicone coatings have achieved

performance levels that may be sufficient for certain military applications. In view

of this information, the Microcircuit Reliability Division of the Rome Air

Development Center (RADC) has entered into participation with the IEEE Computer

Society, Computer Packaging Committee, on the GEL Task Force.

III. GEL TASK FORCE

A consortium has been formed to investigate the state-of-the-art in GEL

coatings. This group has an open membership policy that welcomes participation

from any organization that has a contribution to make to the test program. The list

of participating organizations is attached (Appendix 1). Five meetings have been held

in nine months, plus two test subcommittee meetings. These discussions have

produced a test program composed of: test methods, test plans, and assignments of

responsibilities for achieving the task.

IV. THE TEST PROGRAM

The test program is composed of three test methods, three test plans, and a

detailed logistic flow diagram to chart out the movements of the plastic packages

under test. The three test methods were selected by the test subcommittee. The intent of

these tests is not to perform an extensive qualification of one or more GEL coatings

and plastic packages for military use. The tests were chosen primarily to assess the

coatings' ability to resist corrosion on the die. The first test is thermal shock as per

291
MIL-STD-883, Method 1011.6. The conditions are cycling from -65°C to +150°C with a

five minute dwell at each extreme. The number of cycles will be given in the plan

section that follows. The thermal shock temperature extremes were selected to

match the most severe present military qualification range. The thermal shock

procedure is the first in the test sequence in order to establish a path of ingress

between the environment and the die. This will test for any weakness of the GEL

coating adhesion to the passivated die. The second test is salt spray exposure from

MIL-STD-883, Method 1009.6. The conditions of salt concentration, temperature, and

pH are the ones contained in the test method. The number of hours will be given in

the plan section which follows. The salt spray exposure is the second test performed

in order to provide a ready supply of corroding ions to test the GEL coating integrity,

primarily at the bonding pads where galvanic corrosion is likely to occur. The third is

a steam autoclave exposure. The test condtions are taken from a standard General

Electric plastic parts screening procedure.^

The temperature is 130°C, the relative

humidity is 85% and the parts are under bias during the test. This autoclave

procedure has proven to be the single most consistent screen in assessing organic chip

protection and serves to perform the function of the fine and gross leak tests which

follow environmental test exposure of hermetic devices.

The three separate test plans were formulated in order to assess the validity of

the test methods. It would serve no purpose if the first test destroyed all of the

parts. All the time, money, and energy invested to date would be wasted. The

pretest phase (P.T.) will perform a quick screen of just packaged parts without

printed wiring boards to assure the three tests are appropriate. The P.T. phase

includes 20 cycles of the thermal shock, 2k hours of the salt spray, and the autoclave

test period is 96 hours. No failures are anticipated in these pretests which include

only a small sample of the GELs. If a major failure mechanism appears, the task

force will have to evaluate the materials under test,’ the assembly processes

292
used, and the test severity levels. Assuming no failures in the pretests, a selection of

devices will be mounted to the specially designed printed wiring boards for the initial

test phase (I.T.). This initial test is designed to assess the ability of the printed

wiring boards, board connectors, conformal coating, and solder joints to survive the

test environments. The stress levels for the I.T. phase are the same as for the

pretest phase (20 cycles of shock, 24 hours of salt spray, and 96 hours of autoclave).

The full test phase (F.T.) will follow assuming the boards and parts can survive the I.T

phase. The F.T. will see the thermal cycles increased to 500, the salt spray raised to

96 hours, and the autoclave increased to 168 hours.

The chip test being used in this test is a triple-track resistor with 10 micron

wide lines on 10 micron spaces. The center stripe has a series resistance of about 400

ohms. The resistance of the center track on each chip will be measured after

packaging as well as before and after each environmental exposure. The leakage

current between the center track and each of the parallel neighboring stripes will be

measured also, with a 10 volt potential applied. The bias applied to the center stripe

during the autoclave test will also be 10 volts.

There will be about eight printed wiring boards, each representing a GEL under

test, with 15 experimental and three control packages on them. A set of "golden

devices" will be held at the assembly point to assure all changes in the electrical

measurements are due to the environmental exposures and not sample aging. Failure

analysis will be performed on at least two devices per cell.

V. PRESENT STATUS

The specially designed triple-track resistor chips have been fabricated by Harris

Semiconductor. Premolded cavity plastic packages have been provided by AMP.

Prototype printed wiring boards are provided by Rockwell-Collins. Several candidate

silicone gels will be applied by AT&T.

293
Estimates at this time place the completion of the test phases in the following time

frame: P.T. phase Jan 87; I.T. phase Apr 87; and F.T. phase Dec 87. Results of the

various tests will be made public after the data has been properly compiled.

VI. THE FUTURE

An obvious question that the task force activity raises must be: What would be a new

task force activity? If one assumes for the moment that one or more of the GEL

coatings performs well in the tests, then providers of microcircuits to the military

would be expected to request procedures for qualifying coated devices. In the mean

time, the task force will have provided industry and government a test sequence to

assess non hermetic technology. The task force is discussing the extension of GEL

coating use to injection molded packages. The process simplification derived from

injection molded processes over premolded assembly could be very significant. The

task force considered the testing of injection molded parts but found it would likely

require substantial development time to establish a successful technique to mold over

the GEL coatings. For this reason, the task force is now studying only the premolded

package GEL coated chip option.

VII POSTSCRIPT

The considerations for exploring plastic package use in the military

environment are many. Use of common processes from the commercial sector is an

important one. The fusion of best commercial practices into the military

procurement system could benefit both sectors of the market. Another potential

benefit is better signal speed performance and a closer match between package and

printed wiring board physical properties. Availability, competition between vendors,

and possible faster insertion of newest technology into military equipment could

result. However, the astute reader may have noticed the lack of a certain word or

criteria in this postscript. That word is obviously cost. To understand why a deep

294
reduction in cost is not a likely result of success in using plastic packages, one must

understand where the costs come from. Present commercial use plastic packaged

devices are neither designed nor tested to operate over the full -55°C to +125°C

temperature military environment. In addition to being required to meet tight

electrical performance criteria over this temperature range, military devices must be

able to be cooled conductively by a center core heat sink on the printed wiring board.

In order to be designed to meet the substantially more severe testing and operating

criteria necessary to satisfy military shipping, storage, handling, radiation, flight

avionic, space repair, etc. environments, there may not exist a substantial cost

differential between plastic and hermetic parts.^ The valuable benefits would more

likely be the increased speed performance, weight, and materials compatibility with

the present glass-expoxy substrate technology.

ACKNOWLEDGEMENTS

To Mr. Jack Balde, IDC, the Chairman of the Task Force, who has made it

possible for this consortium approach to work.

To Mr. Marty Freedman, AMP, the Coordinator of the Task Forces' actions.

To Mr. A1 Sasaki, McDonnell Douglas, who has contributed extensively to the

test plan.

295
VIII. REFERENCES

(1) Assessment of Silicone Encapsulation Materials: Screening Techniques; A.

Christou and W. Wilkins, Proceedings of the 1977 IRPS.

(2) Moisture Resistive, Ultraviolet Transmissive Passivation for Plastic

Encapsulated EPROM Devices, K. Alexander, 3. Hicks and T. Soukup, Proceedings of

the 1984 IRPS.

(3) Moisture Protective Coatings for Copper Thick Film Systems, 3. P. Mannitt,

Proceedings of the 1984 ISHM.

(4) Sealing Mechanism of Silicone 3elly Encapsulation with High Reliability, K.

Otsuka, Y. Shirai, K. Okutani,- Proceedings of the 1984 ECC.

(5) C. P. Wong, Private Communication.

(6) Accelerated Corrosion Testing in Pressure Cooker at 130°C, 3. Burgess, A.

Yerman, Proceedings of the 1986 ECC.

(7) IC Quality Grades: Impact on System Reliability and Life Cycle Cost, M. G.

Priore, RADC/RAC Report NB SOAR-3 (1983-1986).

296
APPENDIX 1

Organizational Membership List:

Westinghouse Electric

Hughes Aircraft

Fairchild Camera and Instrument

Microelectronics and Computer Technology Corp

Loral Electronic Systems

McDonnell Douglas

Texas Instruments

Interconnection Decision Consulting

Boeing Electronics

Dow Corning

International Telephone and Telegraph

Rome Air Development Center

National Bureau of Standards

Martin Marietta

Rockwell Avionics

AMP Inc.

General Electric

American Telephone and Telegraph

Harris Semiconductor

Lehigh University

International Business Machine

Naval Ocean Systems Center

Rutgers University

Aerospace Corporation

297
5.3 MOISTURE MONITOR FOR PLASTIC PACKAGED INTEGRATED CIRCUITS

HOWARD L. BLAZ

Harris Semiconductor

P.O.Box 883

Melbourne, FL. 32901

305 724-7128

ABSTRACT

The HARRIS Moisture Chip (H10-55001-6) designed for use in in-situ

determination of internal water vapor content of hermetic integrated

circuit packages can be used to monitor humidity performance of plastic

encapsulated integrated circuits.

Tests run on the moisture cell in different brands of plastic from

multiple assembly lots will repeatably distinguish between materials and

processes

.

This device provides a test structure to evaluate plastic packaging

independent of wafer fabrication related variables. The data is simple

and quickly obtained using inexpensive monitoring of leakage currents

after stressing. Testing is non-destructive so parts can be returned to

the stressing conditions for continued evaluations or until the onset of

corrosion.

The experimental design and a statistical analysis of the data will

be presented. This data demonstrates the effectiveness of this test

structure in determining moisture penetration in different plastic

compounds and the variability in the assembly process.

298
BACKGROUND

Conventional methods of evaluating the moisture resistance of

packaged Semiconductors (temperature-humidity-bias and autoclave) are time

consuming, labor intensive, and expensive. For these reasons the feasibility of using a moisture sensitive test cell was investigated. Moisture

cells used in this study are unpassivated interdigitated aluminum combs on

thermal oxide. The leakage current between interdigitated combs is used

as an indicator of the presence of moisture.

CELL 4 CELL 3

CELL 1 CELL 2

The HI-55001 lias four electrically identical moisture sensing cells on chip. The user may apply any one cell or

parallel combination of cells in a moisture measuring experiment. The in- dividual cells are accessed as follows:

Cell 1 — pads 2 & 3; Cell 2 — pads

4 & 5; Cell 3 - pads 8 & 9; Cell 4 -

pads 10 & 1 1 respectively.

Chip Size: 95 mils by 50 mils.

Copyright (c) Harris Corporation 1979

The current flow to the cell is high initially and diminishes until

the current reaches a steady state which is the leakage current (analogous

to the charging of a capacitor). In this study leakage currents in the

10*13 amp range were observed in unstressed encapsulated cells. Steady

states are attained rapidly enough to make measurements 1 minute after

biasing parts with 5.0 volts. It is believed temperature and humidity

stressing in autoclaves will eventually cause leakage currents to increase

several orders of magnitude. In the following studies, leakage currents

were observed to increase seven orders of magnitude.

299
The interval between test points (time on stress) must be carefully

chosen since the leakage current measured is subject to a series of time

ordered events (See below).

Segment A is zero hour to the onset of leakage current.

Segment B is the period of time in which the leakage current

increases due to addition of electrolyte (ionics and

moisture) to capacitor.

Segment C is the period of time in which the leakage current

decreases due to corrosion (lowering of aluminum

conducti vi ty ).

In addition measurements must be made quickly upon the removal of the

part from stressing. The moisture driven to the cell surface by the

thermal and pressure effects of the autoclave will leave the cell surface

in the absence of these effects. This phenomenon is known as recovery.

It takes place in fewer than 10 hours.

3 00
APPROACH

A preliminary study (Study #1) was conducted to determine a reasonable time for current measurement after removal from stress in order

to reduce the effects of recovery.

Additionally a determination was made of the interval for interrupting stressing to make measurements in order to reduce the effects of

corrosion. Three standard products of plastic molding compound

manufacters were used to encapsulate moisture cells which had been

specially doped with phosphorous in the thermal oxide beneath the

metallization. The phosphorous dopant when combined with moisture

penetrating the plastic would give a significantly higher electrolyte

concentration at the die surface than would have been seen in the typical

moisture cell. This doping made the cell so sensitive to moisture that

all three plastics performed similarly without regard to ionics and/or

lead lock. The time interval from hour zero to the end of segment B (see

above) was approximately 15 hours. This study also confirmed the behavior

of segment C (see above).

A second study was conducted to determine the feasibility of using

the moisture cell to distinguish between different brands of plastic

% used for encapsulation.

A third study was then performed utilizing a statistically designed

experiment to confirm the repeatability and seek to distinguish process

variation. The experimental section discusses this third study.

EXPERIMENTAL

Five standard products of plastic molding compound manufacturers

301
were transfer molded onto 16 lead copper leadframes with moisture cells

(all from same wafer lot) attached (polyimide die attach). This molding

was conducted over a 5 day period with each plastic being 'shot' everyday;

only the order of the 'shots' being varied. Between 'shots' the mold

chase was cleaned with melamine and preconditioned with dummy 'shots' of

the next plastic to be used.

On days 1 and 2 the cel 1/leadframe assemblies were spray cleaned with

isopropyl alcohol (electronics grade) post wire bond. On days 3, 4, and 5

the assemblies were additionally cleaned with isopropyl alcohol prior to

placing them on the hot chase. Molding was done 1 week post wire bond.

Stressing was performed in 24 hour intervals. Parts were grouped in

5 trays of 5 parts each (the five parts having one representative from

each day/plastic combination). The parts were randomized as to position

in the mold chase at time of molding. The order of measuring the parts in

the trays and the order of the trays set for measurement was fully

randomized using the latin square technique to block extraneous variables.

The parts were removed from stress and biased with 5.0 volts on a

Hewlett Packard HP 4145A Semiconductor Parametric Analyser to measure the

leakage current. This device was controlled by a HP 9836 computer to

switch between 3 sets of interdigitated combs on each cell and to record

the data. This setup was used to record zero hour stressing and 16

successive intervals of 24 hour stressing.

Statistical process control (SPC) was applied to the parametric

analyzer using unstressed parts to assure the elevated current readings on

stressed parts were not affected by drift in measurement.

A a =0.39 x = 0.35 pA

302
The samples were stressed in an unbiased condition using a National

autoclave monitored with a Sensotec (SA) pressure transducer and a Doric

(Trendicator 402A) thermocouple monitor. This equipment at 100% relative

humidity (R.H. ), saturated environment cycles on at 13.9 pounds, 120°C and

off at 16.3 pounds, 122°C. Parts were removed from stress, tested/

monitored, and returned to stress within 2 hours.

A general linear models procedure for statistical analysis utilizing

the Bonnferroni (Dunn) T. tests for the variable log of measured current

was applied to the data. The computer program used was S.A.S. (Version

79.6; SAS Institute Inc., Box 8000, Cary, N.C. 27511).

RESULTS AND FINDINGS

The following table contains the results of the studies.

Stressing Periods to 3rd Study 2nd Study

achieve elevated (statistically designed)

leakage current

Fewest (worst performance) Compound A Compound. C Compound B Compound A

Compound C Compound B Compound D Compound D Most (best performance) Compound E

NOTE: 1) Compound E was not available for 2nd study.

2) Assembly lot variation of Compound C performance

is consistant with historical reliability

data gathered by Harris Semiconductor.

303
The following graph contains the time-leakage current relationship

of the statistically designed (3rd) study.

Additionally, those parts subjected to isopropyl alcohol rinse

immediate prior to molding (indicated by *) showed a trend toward lower

leakage currents than those not so rinsed.

(Hours)

30.4
Position in the mold chase did not indicate any repeatable variance

in moisture cell performance.

The onset of elevation of leakage current was a function of handling

of stressed parts. The use of fingercots extended the time of the

experiment from 5 days (2nd study) to 16 days (3rd study), demonstrating

the effects of 'clean' handling. Further invesigations are planned to

explain this phenomenon in detail. It is not known at this time if

moisture ingress was by bulk diffusion or through the leadframe-plastic

interface. It is not known at this time if the ionic content of the

electrolyte reflects material solely available in the encapsulating

plastic.

CONCLUSIONS

The interdigitated comb moisture cell provides a test vehicle for

plastic mold compound T&H performance comparison. The response of the

cell to stressing is repeatable. Measurements are performed utilizing any

ammeter with a pA range.

The use of this cell as a performance probe eliminates lot to lot

and/or wafer to wafer variability on production integrated circuits as a

source of variation in a controlled experiment. Data can be gathered

using simple equipment eliminating variability due to test handler

situation. Parts need not be*stressed to catastrophic failure. They can

be stressed to the attainment of an arbitrarily determined leakage

current; a great time saving.

Additionally, stressing can be accomplished in a relatively

inexpensive autoclave chamber without bias. This will allow comparisons

305
in material and/or process performance to be evaluated. Stressing times

are considerably less than in 85°C/85% R.H. chambers. At this time the

issue of correlation with actual device life performance has not been

investigated.

Lastly, the lack of passivation of this die allows it to be used as

a strain gauge. Decapsulation should reveal shifting of portions of the

aluminum interdigi tated comb in high mechanical stress applications.

Experiments are currently underway to assess this potential usage. .

ACKNOWLEDGEMENTS

W. L. Schultz and H. A. Swasey for encouraging this work.

D. M. Andrews for encouraging my interest in semiconductor packaging.

306
5.4 HIGH RELIABILITY MOLDED PLASTIC PACKAGING (I)

J

THROUGH A SYSTEMATIC APPROACH

LOUIS LIANG

LINEAR DIVISION

FAIRCHILD SEMICONDUCTOR CORP

313 Fairchild Drive

MOUNTAIN VIEW, CALIF 94039

I. INTRODUCTION

Traditionally plastic packages are known to be susceptible to

moisture induced failures. This is particularly worrisome with

some SMD such as SOIC which are subjected to severe temperature

shock during board soldering. [1-3]

The industry customarily defines failure as non-recoverable

corrosive defect on the chip. Test methods commonly used today

are static pressure pot at 1 ATM; bias pressure pot from 2-8

ATM; and 85/85 THB. The user community, with the exception of

only a few, specifies static pressure pot as the standard tool

for HAST. The requirements for time to failure has been

steadily increasing: from 24 HR in the early 70's to 96 HR in

the early 80's; and now 168 HR appears to be a common

requirement. Some users have begun to demand bias pressure pot

as an incoming inspection method.

In order to meet the ever more stringent requirements and

possible future acceptance of plastic packages in certain

military applications, most semiconductor manufacturers have

active programs to improve reliability performance of their

plastic packages as a marketing edge. Some major IC producers

even have programs addressing issues far beyond the present

commercial applications. This report, however, is limited to

the presentation of a "high reliability" conventional molded

plastic package for commercial application. It should not be

construed in any way to compare with "Gel Coated" plastic

packages from AT&T, GM, Hitachi, etc. [4]

II. HIGH RELIABILITY DEFINITION FOR MOLDED PLASTIC PACKAGES

There is obviously no agreed upon industry standard for plastic

packages reliability. Time to failure performance for pressure

pot also varies significantly from vendor to vendor. In order to

establish a reliability performance measurement, Fairchild's

Linear Division has instituted a phase objective program as

shown in Table I.

307
TABLE I

FAIRCHILD LINEAR DIVISION MOLDED PLASTIC PACKAGE

RELIABILITY DEFINITION FOR MOSITURE RESISTANCE

(INTERNAL BENCH MARK FOR PRODUCTION PARTS)

TEST METHOD OLD ( pkg

)

REL I '85 REL II REL III

STATIC PR POT *1

96 HR 0/45 __ 168 HR 2/45 0/45 -- -- 336 HR -- 0/45 -- -- 500 HR -- 1/45 0/45 -- 1000 HR -- 2/45 0/45 -- 2000 HR — "" 0/45 0/45

BIAS PR POT *2

48 HR 0/15

168 HR -- 0/15 -- -- 500 HR -- 0/15

1000 — — 1/15 0/15

*3

PARALLEL TESTS YES YES YES YES

*3

SEQUENTIAL TESTS NO NO YES YES

85/85 THB *2

1000 HR < 1.0

2000 HR — — < 0.1 0 0

CORROSION YES- YES YES YES

CRITERIA

PARAMETRIC NO NO YES YES

CRITERIA

« e ...

*1 Production Monitor Performed Twice Weekly.

*2 Reliability Monitor Performed Regularly.

*3 Thermal Shock; (Salt Spray); Autoclave or- 85/85 THB

308
III. INVESTIGATIONS

In order to fully understand moisture induced failure mechanisms

in conventional molded plastic package, detail experimental matrix

was used to identify moisture path, contaminants, process

conditions, design rules, and test methodology.

MOISTURE PATH

As shown on Figures 1 and 2 and photo slides. [5,6]

Key contributors to moisture path in terms of severity were found

to be Trim/Form; Leadframe Design; Leadframe Material; Plating

Material; Molding Compound; Lead Finish Operations; Flux; and

Deflashing.

COMPATIBILITY OF MATERIALS

Contaminants in a molded plastic package were traced to three

major sources; Flux Residual; Molding Compound; and Die Attach

Paste. Although impurity level in the materials described above

are crucial factors [7,8], material compatibility was found to be

very important as shown in Figure 3. It was also found, during

the final phase of the material selection process which involved a 3X3 matrix for die paste and molding compound materials, that 2

out of 9 elements consistently had superior results. Search of

published literature however failed to uncover any similar report

on this interesting and important phenomenon.

PROCESS CONDITIONS AND DESIGN RULE

To take maximum advantage of modern low impurity packaging

materials and halide free flux, it was necessary to pay

significant attention to design rules [9] (both chip layout and

leadframe design) , and assembly process conditions. Table II

shows where improvements were made in the new plastic package at

Fairchild* s Linear Division in order to achieve the reported

performance.

TABLE II. PROCESS IMPROVEMENT SUMMARY

AREA OF IMPROVEMENT DESCRIPTION

Die Size Vs Pad Size

Wire Bond

Plastic Locks and Moisture Trap

Mold Grating

Trim/Form

Deflash Process

Lead Finish

Parts Handling

Maximum die size defined.

Max length; loop height; and wire

angle from bond pad to finger

defined.

Incorporated.

Redesigned.

Redesigned.

Changed.

Re-tooled and Re-vamped.

Eliminated all manual contacts

with parts.

309
TEST METHODOLOGY AND PLASTIC MATERIAL QUALITY

A rigid test plan was found to be essential for post moisture

environment tests because parametric shifts in some high precision Op

Amp devices. They were, however, not at all apparent in logic and

interface devices tested. Generally parametric shift and low level

leakage were recoverable with time and may be a fact of life for high

precision Linear devices in today's molded plastic packages. Fig. 4

shows a typical recovery curve for a sensitive high precision Op Amp

Packaging material consistency is another key variable for recoverable

parametric failure (Fig. 5). Material supplier's ability to maintain

quality consistency cost effectively . in a high volume production

environment, and our ability to implement a fast and effective incoming

quality inspection method are yet to be developed.

Fig 4. Recovery Curve of an Op Amp

310
IV. RELIABILITY PERFORMANCE SUMMARY

TABLE III. PERFORMANCE COMPARISON OF "OLD" AND "NEW" PACKAGES (OP AMP

TEST COND. OLD (CONTROL) NEW (HI REL I)

ENGR. PROD (MON) ENGR PROD (MON)

OP LIFE

125 ' C 1000 HR .22% 0.7% 0/1000 + 0%

2000 HR NA NA 0/1000 + 0%

85/85 THB : 1000 HR .8% 3.3% 0/1000 0%

2000 HR NA NA 0/1000 0.05%

121’ C PR POT

168 HR 2.0% 3.4% 0/6000 0%

500 HR NA NA 0/6000 0.01%

1000 HR 1/6000 0.15%

BIAS PR POT

145 ' C 85% RH: **

96 HR 1/89 0/209

168 HR 0/88 0/207

500 HR NA NA 3/88 0/207 NA

700 HR 11/85 0/207

1000 HR 0/74 0/207

THERMAL SHOCK

-55 ' C - 125 ' C

500CY 0% 0% 0/1000 0%

1500CY NA NA 0/100 NA

TEMP CYCLE

-65 ' C - 155 '

500CY 0% 0% (6/500)0/100 0%

1500CY NA NA 0/100 NA

SOLDERABILITY

1 HR ; 0% 0% 0% 0%

) 16 HR : NA NA 2/50 NA

REMARKS: Interface and logic devices typically have much better

performance than precision Op Amp.

** Interface device.

311
V. DISCUSSION. AND. CONCLUSION

With modern low impurity plastic packaging materials, it is now

possible to produce high reliability (I) molded plastic package

for commercial applications. However, material technology alone

is far from adequate, A systematic approach encompassing design,

process, and carefully matching packaging materials was

necessary to obtain the high performance reported. The biggest

challenge, to date, is our ability to implement effective

process control in production environment to hold the gain

consistently without having to resort to exotic total automation

in material manufacturing and package assembly.

The audience must be reminded that conventional molded plastic

package may never achieve the same "hermeticity " performance we

have so taken for granted with ceramic packages. This may be

particularly true for some extremely sensitive high precision

Linear devices. There is still a long road ahead of us before we

are capable of producing a "High Reliability III" molded plastic

package which could be universally acceptable by military

applications while still being cost competitive with the present

commercial molded plastic package. We may need to develop new

passivation and molding technology in order to reach the next

plateau. We are, nevertheless, encouraged by the preliminary

results of our "Rel II" development project that cost effective

solution is not entirely elusive.

ACKNOWLEDGEMENT

The author wishes to thank A. Sezen and J. Taylor for performing

almost all the tests and failure analysis in the study; G. Chu

and Linear's Korea Assembly and Test Plant for preparing many

thousands of samples and data analysis used in this report.

REFERENCES

1. L. Feistein, "Failure Mechanism in Molded Microelectronic

Packages"; Int'l Microelectronics Conf pp. 49-57, (1979)

2. R. Larson, "A Review of the Status of Plastic Encapsulated Semi.

Component Reliability"; Br. Telecom Tech J. V.2, No. 2, (1984)

3. S. Ito, et. al, "Special Properites of Molding Compound for SO

Packaged Devices" ECC (36) Proc, PP. 360-365, (1986)

4 . A. Otsuka, et. al, "A new Gel Sealing Mechanism for High

Reliability Encapsulation"; IEEE CHMT-7 , No. 3 , PP. 249-256 (1984)

5. M. Murtuea, et. al "Flux Penetration and Pressure Cooker Fail

Mechanism in Plastic IC Packages" ECC (36) PP. 616-621 (1986)

6. R. Stroehle, "Influence of the Chip Temp, on the Moisture

Induced Failure for Assessing Plastic Encapsulated Semi.

Components"; IEEE CHMT-6 , No. 4, pp. 537-543 (1983)

7. M. Iannuzzi, "Reliability and Failure Mechanisms of Nonhermetic

Aluminum SIC's: Literature Review and Bias Humidity Performance"

IEEE CHMT-6, No. 2, pp. 181-201, (1984)

8 . W. Collins, et. al, "Improved Moisture Resistance in Plastic

Package" ECC (35) PP. 14-17, (1985)

9 . R. Thomas, "Stress Induced Deformation of Al. Metal ization in

Plastic Molded Semi. Devices"; CHMT-8, No. 4, pp. 427-439, (1985)

312
Fig

1.

Moisture

Path

ina

SOIC

Package

<u 4-> o> > -O

<v ia

s- <D o _i

vO <0 o o

*ri

m o o n n

«o vO o o m m o o•

313 J
FAIRCHILD

A SchlumtMrgftf Company

Fig 2. Flux Residual in a Plastic Dip

314
A

Schlumbaigai

Company

315
FAIRCHILD

A Schiumb*>g*f Company

September 24, 1984

PRESSURE POT 121 DEG C

CUMULATIVE PERCENT FAILURE RATE

VS.

WEEKS IN PRESSURE POT

Combo 1 Combo 2 Control — Ni Control — Cu

Nickel Nickel

PERCENT

Fig 3. Packaging Material Compatibility
FAIRCHILD

3T7

5.5 HIGH LEAD COUNT / HIGH RELIABILITY

PLASTIC PACKAGING:

ISSUES AND APPROACHES

Greg Pitts

Microelectronics and Computer Technology Corporation

12100 Technology Blvd.

Austin, TX 78727

(51 2)-331 -6200

Abstract

Microelectronics and Computer Technology Corporation (MCC) is a research consortium

developing both multi-chip and single chip packaging technology for high speed, high lead count

integrated circuits on behalf of ten major U.S. companies. Approaches for both plastic and

hermetic packaging in single chip applications are being developed.

The research in plastic technology is aimed at addressing applications from low lead count to those in excess of 300 I/O. The reliability of these packages depends upon a number of factors,

including die size and moisture sensitivity, leadframe and package design, molding compound,

material properties, and processing method and parameters. Unfortunately, the larger the die

size and the higher the lead count, the more difficult it becomes to meet high reliability

requirements.

This paper will describe some of the experiences with these and other factors. It will also

outline some of the research and testing steps which will be needed before high lead count

plastic packages can be considered for demanding commercial and military environments.

Introduction

The trend in semiconductor manufacturing has been towards integrating more and more

functions into the same area of silicon. Unfortunately, the advances in integrated circuit

manufacturing have outgrown the industry's ability to package these new devices effectively.

The demands are for better electrical and environmental performance. At the same time, the

silicon size and I/O are increasing, and the package body and lead pitch are decreasing.

The basic issues to be addressed, however, are not lead count dependent. The requirements of the

commercial, industrial and military markets are not lead count specific. The same factors that are important in packaging low lead count devices are simply amplified by the smaller spaces

between the leads and large die size. The desire to shrink the package while increasing the

silicon area means that there is less plastic to protect the device. As a result, plastic packages

are much more likely to crack and permit rapid moisture and ionic contaminant ingression.

Even in a "good" package, the moisture path is shorter than with a conventional DIP because of

the thinner walls. As the pitch between the leads gets tighter, the lead thickness gets thinner

and manufacturability suffers due to handling problems.

These trends toward higher lead count and smaller packages make it more difficult to achieve

reliability, but reliability is relative to the application. Reliability implies longevity, the

ability to perform as specified in an appropriate environment for a given length of time. But

319
references to "high reliability" are generally understood to be synonymous with extreme

military requirements, i.e. meeting the requirements of MIL-STD-883 and the likes. These

specifications call for survival in severe environments with wide temperature extremes,

situations in which plastic packages have not performed well. Plastic packages are highly

reliable, but to date that has been limited to commercial and industrial type environments.

Current Status

Acceptance of plastic packages into severe military environments requires an extension of the

current industrial testing beyond current levels. Typically, commercial applications require

more cycles of a particular test, but with relaxed conditions. For example, in temperature

cycling, the test may be from -40° C to +70° C for 1000 cycles instead of -55° C to 125° C

for 10 cycles. In a sense then, the required reliability of the commercial environment is more

severe than the military in the case of extended cycling. It is the low end of the cycle, the

excursion below -40° C, that is particularly difficult for a plastic package to survive 'without

cracking.

The difficulty in meeting military requirements comes not from the extended cycling but from

the greater temperature extremes of the environment and harshness of the atmosphere.

Conventional thermoset molding materials have a thermal expansion rate significantly higher

than silicon and impart a great deal of stress to the silicon device and it's attachments. Any lack

of cleanliness in the process or impurity in the molding compound which provides mobile ionic

species will lead to corrosion with the ingress of moisture into the plastic. But although some

new materials and processes show some promise in alleviating these problems, there are no

specifications written for them to meet these "high reliability" requirements.

What is needed is a specification similar in scope to MIL-STD-883 and MIL-STD-3851 0 for

plastic packages. The demands of a specification such as this would of course be specific to plastics and would need some unique features. At present, it is not possible to get sufficient

feedback during the manufacturing process about the reliability of the package. There is no non- destructive test equivalent to the seal test for hermetic packages. Pressure cooker,

temperature cycling, and other accelerated tests are destructive and time consuming in nature. Because of this, the most effective way to control the reliability of plastic packages is through

emphasis on control of processes and materials, including incoming inspection. A

comprehensive specification for plastic packages should require these controls.

Primary Packaging Concerns

There are a number of technical considerations in producing a reliable plastic package,

particularly in attempting to meet the severe environment requirements of a military type

application. The four most important concerns—device susceptibility, package design, material

control, and process control—are described below:

Device Susceptibility

The most significant failure mechanism with plastic packages is corrosion. Conventional

devices have exposed aluminum bond pads with only microns of thickness. Stress relief may

come in the form of passivation cracks which expose the underlying metallization. In the

presence of moisture and mobile ions, such as Na++, K++. Ch or Ca++. electrochemical cells

320
are formed during use which can destroy the metallization and the functionality of the device.

The trend today is towards producing a "bulletproof" structure that is resistant to moisture and

corrosion. Passivation technology, using plasma nitride and other approaches, has improved

the reliability of devices through increased crack resistance and elimination of pinholes.

More important, however, is the use of a bumping technology to protect the bonding areas. One

way to do this is to use gold bumps with a barrier layer between the gold and the underlying pad

metallization. Gold is much less susceptible to corrosion than aluminum. Bumps are also

typically on the order of 1 mil in thickness, much thicker than the aluminum pad it is protecting which also enhances survivability. This type of bump is most often used for TAB

(tape automated bonding). Other systems used for TAB include solder, where solder bumps are reflowed to hold the lead, and copper, both of which provide the same kind of volume

improvement and protection of the aluminum as does gold.

The intended result in combining a state of the art passivation technique with bumping

technology is the creation of a "hermetic" device, one which doesn't have the susceptibility to corrosion whether packaged or not. This would relieve much of the requirement of the package

to protect the device. The package could then become a vehicle for mechanical protection.

Package Design

The package design becomes more significant as the die gets larger and the package becomes

smaller while the lead pitch gets tighter. Intelligent decisions must be made to keep the

dimensions within reason. Equally important, the selection of appropriate molding materials

and manufacturing processes is critical. Plastic materials shrink upon cooling and can distort

lead geometries, particularly important for the smaller leads found in high lead count devices.

With more fragile leads coming out of the package, a scheme for maintaining the integrity of

these leads is mandatory.

Material Control

Plastic materials with the appropriate properties must be developed and maintained. A

thorough knowledge of the parameters which influence reliability is necessary so that a dependable incoming inspection procedure can be developed. Since it is impossible to monitor

"reliability" during manufacture, lot to lot variations must be controlled within defined limits

through incoming inspection testing and supplier process control. Of particular concern is the

amount of ionic impurities inherent in the molding material, lot by lot.

Process Control

Process control is an equally as important factor in maintaining high reliability in plastic

packages. There are no 100 % screening test for plastic packaged parts, no non-destructive

tests that can be used to weed out the potentially defective parts. Plastic parts can be

destructively evaluated on a sample basis only and these tests are typically time consuming.

There is no room for a manufacturing process to be out of control until reliability predictions are made on manufactured packages. Reliability must be built in, not tested and inspected in.

321
MCC Concept

This research and development consortium was founded to cooperatively advance the state of the

art of the U.S. computer industry. Of the four major programs in progress at MCC, three of

them are aimed directly at computer technology, software and architecture. The fourth, the

Packaging/Interconnect program, is concerned with the component and sub-system level

hardware and the results, although proprietary, are applicable to the electronics industry in general. Funding for each of the programs is independent, so the participation in each specific

program is unique. The Packaging/Interconnect program currently has ten shareholders,

which are the following:

MCC Efforts

Generate a High Reliability Package Concept Incorporating TAB Technology

MCC has developed a plastic package concept which is based on the use of TAB instead of wire

bonding. In this package, the volume ratio of plastic to die has been reduced from 90:1 for a conventional 40 lead DIP to 11:1 while providing the capability of 300 or more I/O.

TAB technology has been around for quite some time, but never fully utilized for high lead count

devices. Only recently has the industry embraced TAB and attempted to put it into volume

production. There are a number of performance issues which are driving this acceptance of

TAB, including improved electrical performance. It is the mechanical features, however, which

are relevant to reliability.

Bonds formed with a TAB process have significantly increased bond pull strengths over wire

bonds. Typical bond pull strengths are greater than 30 to 40 grams, whereas comparable wire

bond pulls over 15 grams are unusual. In addition, the incorporation of TAB allows the

elimination of one interconnect level by using the TAB tape as the leadframe. In this situation,

the lead which bonds to the device is also the lead soldered to the substrate. When these features are combined with state of the art bumping and passivation technology, reliability of a plastic

package can be significantly improved.

In order for this concept to be applicable to high lead counts, the problem of thermoset material

flashing must be addressed. Thermoset materials have very high flow properties and will pass

through openings as small as 0.5 mils (0.0005"). This means that material will flow in between the leads and beyond the desired package body. In the conventional package, this is controlled through the use of dam bars, metal strips shorting the leads together. These strips

prevent the plastic from being extruded between the leads and are removed in subsequent steps.

With high I/O packages, the spacing between the leads becomes very small, potentially less than even 5 mils. In this case, it becomes uneconomical if not impossible to remove the dam bars

mechanically. It would be better to eliminate the need for them.

Advanced Micro Devices

Allied

Boeing

Control Data Corp.

Digital Equipment Corp.

Harris

Hewlett-Packard

Kodak

3 M

Unisys (Sperry-Burroughs)

322
Center on a Process Geared toward full Automation

TAB has been identified as an approach which lends itself well to automation and can be

processed in a reel to reel format with little human intervention. It has been criticized for lack

of flexibility, but this can be overcome, however, by developing tape families and normalizing

the locations of bond pads. In this fashion, many different devices can be bonded using the same

tape, minimizing set up times and facilitating high product mix situations.

Strive to Eliminate Manufacturing Steps

By using TAB, the tape becomes the leadframe and the bond wires are eliminated. In addition,

there is no need for a die paddle to hold the die in position. Correct die position is provided

during the inner lead bonding process when the die is presented on a pedestal. The inherent

stiffness of the leads eliminates the need for additional mechanical support so the die attach

process can be eliminated except when needed for electrical or thermal reasons.

Dejunk and deflash processes are where the excess plastic and resin material from molding are removed. These steps are required in conventional molding operations because of the high flow

properties of the thermoset materials. Wire bonds are very fragile, so if the materials were

not high flow (low viscosity), wire sweep would damage the device. With TAB, the structure is more rigid, and the interconnect is all in one plane and therefore much less likely to suffer

damage from the molding process. As a result, new materials have been and can be developed to take advantage of this situation which may not require dejunk, deflash or dam bar removal,

because there may not be any dam bars to remove.

Investigate Material Options

In addition to the opportunities that TAB affords (as mentioned above), something must be done

to reduce the stress inherent in plastic packaging. Molding occurs at high temperatures,

typically around 170° C. At this point, the package could be considered stress free. At any

temperature below this point, stress will build up on the device due to the plastic shrinking

faster than the silicon. The worst case, then, is the low end of a temperature cycle, and most

plastics aren't cycled below -40° C (while the military requires -55° C). With the devices

growing in size, this problem is amplified and new material options must be pursued.

Mechanical stress is a result of a combination of factors, the most important of which are the

differential thermal expansion and the elastic modulus (Young's modulus). Stress can therefore

be controlled by developing a material with a lower expansion coefficient, preferably as low as

the thermal expansion of silicon, around 5 ppm/0 C. An alternate approach is to modify the

elastic modulus so that the differential expansion does not result in stress. Typical thermosets

have an elastic modulus of around 2 x 10 6 PSI. An elastomeric material or a thermoset

combined with an elastomeric can have a much lower modulus and impart less stress. The

problem with many of the elastomers is that although they have a lower modulus, their thermal

expansion rate is much higher and stress buildup still occurs.

The other potential opportunity is through the use of thermoplastic materials which are

injection molded. Thermoplastic materials have not been used in the past for a number of reasons. Most thermoplastics do not have the high temperature stability required for

electronics applications. In addition, there have been very few materials with a low enough
melt viscosity to not exhibit excessive wire sweep. A few "highly engineered" thermoplastics

have improved the situation; Phillips RytonTM and liquid crystal polymers from Celanese and

Dartco are available specifically for semiconductor molding. The Phillips material, chemically

known as polyphenylene sulfide has been around several years but has not been fully explored as of yet, particularly for TAB applications. The liquid crystal polymers look very good on

paper. They exhibit very low melt viscosities despite fairly high molecular weights because the

molecules orient themselves in the flow direction. The thermal expansion rate of these

materials has the potential to be the lowest of any known molding material, at least in the axis

of flow.

Develop Molding Processes

Conventional transfer molding of semiconductors requires the use of a leadframe with dam bars.

Although the need for these may be eliminated with material modifications, it is conceivable to

alter the molding process so that dam bars are not needed. In the past, attempts were made to use compliant materials within the mold to seal around the leadframe. The main criticism of

this approach is that the seals don't seem to last very long. Grooving the mold presents

tolerancing problems, particularly with high lead counts. Perhaps using a laser would simplify

the mechanical problems of removing the dam bars.

Another likely approach is to use dam bars that are non-conductive and therefore would not need

to be removed. With TAB, the leads are typically held in place by a polyimide film. Without

this film, the leads could not span any reasonable length. Unfortunately, this polyimide is not desirable inside of a plastic package. Instead, an epoxy window frame could be used which would

double as the support for the leads and provide the seal for the mold. Since it is of comparable

material, if not identical to the molding material, it should not be incompatible with the package

and would not be removed.

The use of thermoplastic materials opens up the possible processing options. Thermoplastics

are injection molded, a more common molding method outside the semiconductor industry. The

equipment available for injection molding today offers significantly higher speed as well as

greater control of the process through closed loop feedback of the molding parameters.

Thermoplastic materials do not require a curing step, so cycle times can be much faster, down

to 20-30 seconds instead of several minutes. Tooling expenses can be minimized while

maintaining the same throughput as a conventional thermoset transfer process since fewer

cavities are needed.

Verification of Concepts

An internal specification has been generated, based on inputs from the MCC shareholders, for verifying the feasibility of these concepts. It includes much of the type of testing seen in MILSTD-883 and is intended to stress the plastic packages to the same levels as would be required

of a hermetic package for the military, if not more severe. Incorporation of TAB and state of the

art materials and processes can accomplish this 'goal.

Additional Research Needed

A great deal of research is required before many of these concepts and approaches will be seen

in the field. It is possible today, however, to build devices in the front end that cannot be

324
packaged and therefore cannot be used, so the need to make improvements in packaging is real. The following identifies some of the more obvious areas:

Material Studies, Development and Characterization

More materials need to be evaluated, particularly the new exotic materials that may offer

significant improvements over conventional thermosets. The thermoplastics mentioned above

show some promise, but they are only a starting point. Thermoset materials are continually

improving, although typically not in leaps and bounds. Careful characterization of any new

material will be required to develop comprehensive incoming specifications.

Process Development and Evaluation

The emphasis needs to be on fully automatable processes for two reasons: reduced high volume

manufacturing cost and control of the process. A more thorough investigation of injection

molding is necessary before this process can be fully utilized in a production environment, with

the limits of the processing window well defined. Other approaches such as compression

molding, or reaction injection molding (RIM) may also become applicable with some of the new

exotic materials.

Definition and Implementation of True Statistical Process Control

The key to maintaining low failure rates in plastic packages is in effective use of process

control. The lack of immediate feedback on reliability and the inability to screen plastic

packages 100% with non-destructive testing requires that the process be under control. The

definition and implementation of a true statistical process control procedure is essential.

Development of New Test Methods for Plastic Packages

New tests methods are needed to provide rapid feedback on reliability for the most effective

process control. The incorporation of HAST (Highly Accelerated Stress Test) as an accelerated

autoclave test would at least speed the return of reliability data, but again, this is a destructive

test. Effort should be expended to develop effective, non-destructive test methods.

Generation of the Standards and Specifications for Plastics for Harsh Environments

A cooperative effort is required to generate the standards for plastic packages for military type

harsh environments. A great deal of field testing and documentation of the test methods and

conditions will assure reliability. To be effective, this will have to go beyond the typical scope

of a qualification specification, and incorporate manufacturing process control practices.

325
6:1 List of Speakers

RADC/NBS Workshop

November 12-14, 1986

M. Andrews

Microelectronics and Computer

Technology Corp.

12100 Technology Blvd.

Austin, TX 78727

(512) 331-6200

Aaron DerMarderosian*

Raytheon Co.

Equipment Division

Sudbury, MA 01776

(617) 440-2791

N. K. Annamalai

RADC/ESR

Hanscom AFB, MA 01731

(617) 377-3047

Bill Evan

WEB Technology, Inc. Dallas, TX

Lyle E. Bergquist

Martin Marietta Denver Aerospace

Dept. 0560, P.O. Box 179

Denver, CO 80201

(303) 977-4519

V. Fong

Panametrics

221 Crescent Street Waltham, MA 02254

(617) 288-2719

E. C. Blackburn

Rome Air Development Center

Mail Stop RBRE

Griffiss AFB, NY 13441-5700

(315) 330-4029

D.Y. Guan

IBM

Dept. A28, Bldg. 908

P.O. Box 950

Poughkeepsie, NY 12602

(914) 732-7238

H. L. Blaz

Harris Semiconductor

P.O. Box 883

Melbourne, FL 32901

(305) 727-7128

T.F. Gukelberger

IBM

Dept. A28, Bldg. 908

P.O. Box 950

Poughkeepsie, NY 12602

(914) 732-7238

E. C. Cahoon

IBM

Dept. A28, Bldg. 908

P. O. Box 950

Poughkeepsie, NY 12602

(914) 732-7238

Ray F. Haack

California Institute of Technology/JPL

M/S 125-214

4800 Oak Grove Drive

Pasadena, CA 91109

(818) 854-6568

D. R. Carley

RCA/Solid State Division

Somerville, NJ 08876

(201) 685-6465

Orjan Hallberg

RIFA AB

S16S81

Stockholm, Sweden S1681

(08) 757-4626

* Session Chairman

826
Karin Hulten

MIMX, IC-Division, RIFA AB

IsaJQordsgatan 10-16, Kista

S- 16381 Stockholm, Sweden

(08) 747-4920

Benjamin Moore

Rome Air Development Center

RBRE

Griffiss AFB, NY 13440-5770

(315) 330-4055

M.S.R. Islam

Clarkson University

Potsdam, NY 13676

R.W. Nearhoff

RCA/Solid State Division

Route 202

Somerville, NY 08876

(201) 685-6465

T.W. Joseph

IBM

Dept. A28, Bldg. 908

P.O. Box 950

Poughkeepsie, NY 12602

(914) 732-7238

John Pernicka

Pernicka Corp.

112 Racquette Drive

Fort Collins, CO 80524

(303) 224-0220

Didier Kane

Rome Air Development Center

RBRE

Griffiss AFB, NY 13441-5700

(315) 330-4055

David Pinsky

Raytheon Company

Hartwell Road

Bedford, MA 01730

(617) 274-4252

Louis Liang

Fairchild Semiconductor

313 Fairchild Drive; M/S 4-390

Mtn. View, CA 94039

(415) 962-4164 or 4166

G. Pitts

MCC

12100 Technology Blvd.

Austin, TX 78727

(512) 331-6200

Robert K. Lowry*

Harris Semiconductor

Box 883, MS 62-07

Melbourne, FL 32901

(305) 724-7566

L.J. Rigby

STC Technology Limited

London Road, Harlow, Essex, CM17 9NA

United Kingdom

A. Mantz

Spectra-Physics, Inc. Laser Analytics Division

25 Wiggins Avenue

Bedford, MA 01730

Stephen R. Schertz

Martin Marietta Denver Aerospace

Dept. 0560, P.O. Box 179

Denver, CO 80201

(303) 977-4519

327
Stephen D. Senturia* R.J. Straub

Massachusetts Institute of Technology Delco Electronics Corp.

77 Massachusetts Avenue, Room 39-567 1800 E. Lincoln

Cambridge, MA 02139 Kokomo, IN 46902

(617) 253-6869 (317) 451-7027

J.M. Snowdon Philip R. Troyk

IBM BT

Dept. A28, Bldg. 908 10 W. 32nd Street P.O. Box 950 Chicago, 11 60616

Poughkeepsie, NY 12602

(914) 732-7238

(312) 567-5324

J. Sproul D. Wall

Spectra-Physics, Inc. Spectra-Physics, Inc. Laser Analytics Division Laser Analytics Division

25 Wiggins Avenue 25 Wiggins Avenue

Bedford, MA 01730 Bedford, MA 01730

328
6.2 Attendees to RADC/NBS Moisture Measurement Workshop

Mr. Don Allen

Motorola

Mail Drop HI 02 3 8201 E. McDowell Road

Scpttsdale, AZ 85252

(602) 949-2386

Mr. Naga Annamalai

RADC/ESR

Hanscom AFB, MA 01731

(617) 377-3047

Mr. Dan Anderson

Bendix

1 721 SE 5th Terrace

Lees Summit, MO 64063

(816) 524-9610

Dr. James Anderson

Ford Motor Company

Scientific Research Staff

Dearborn, MI 48121

(31 3) 594-1 187

Mr. David L. Angst

AT&T Bell Laboratories

555 Union Blvd.

Allentown, PA 18103

(215) 439-5676

Mr. John L. Barchi

Cordis Corporation

10555 W. Flagler Street

Miami, FL 33172

(305) 551-2520

Mr. Richard Benson

Johns Hopkins Applied Physics Lab

Johns Hopkins Road

Laurel, MD 20707

(301) 953-6241

Mr. Lyle E. Bergquist

Martin Marietta Denver Aerospace

Dept. 0560, P.0. Box 179

Denver, CO 80201

(303) 977-4519

Mr. Eugene C. Blackburn

Rome Air Development Center

Mail Stop RBRE

Griffiss AFB, NY 13441-5700

(315) 330-4029

Mr. Colin B. Blakemore

Du Pont Company

Glasgow Site

Wilmington, DE 19898

(302) 453-2382

Mr. Daryl R. Brunswick

Allied Bendix

K.C. Division D/834 2A36

P.O. Box 419159

Kansas City, MO 64141-6154

(816) 997-4328

Mr. Stephen Bryant

Fairchild Inc.

20301 Century Blvd.

Germantown, MD 20879

(301) 428-6170

Mr. Stephen P. Carve lias

Gollob Analytical Service

47 Industrial. Road

Beckeley Heights, NJ 07922

(201) 464-3331

Mr. Bill Collins

M&T Chemicals

P.O. Box 1104

Rahway, NJ 07065

(201) 499-2534

Mr. Kevin D. Cluff

Sperry Corporation

M/S L24C4

21111 N. 19th Avenue

Phoenix, AZ 85027

(602) 869-2869

Mr. David J. Daley

I.T. Corp. , Cerritos

17603 Fabrica Way

Cerritos, CA 90701

(213) 921-9831

329
Mr. Ray Denton

Mail Code 6051

Naval Weapons Support Center

Crane, IN 47522

(812) 854-3828)

Ms. P. Jane Gale

RCA Laboratories

P.O. Box 432

Princeton, NJ 08543-0432

(609) 734-3053

Mr. Robert Edelman

M&T Chemicals

Randolph Avenue

Rahway, NJ 07901

(201) 499-2316

Mr. Richard Gerber

Aerospace Corp.

Box 92957, M4/996

Los Angeles, CA 90009

(213) 336-1535

Mr. William Ege

Struthers-Dunn, Inc.

Lambs Road

Pitman, NJ 08071

(609) 589-7500

Mr. Graham Gibson

VG Instruments

4 Elena Court

Medford, NJ 08055

(609) 654-1862

Mr. Edward Etess

WEB Technology, Inc.

10520 Plaro Road, Suite 104

Dallas, TX 75238

(214) 343-9238

Mr. John F. Graves

Allied/Bendix Communications Division

1300 E. Joppa Road

Towson, MD 21204

(301) 583-4133

Mr. John E. Fennimore

Martin Marietta Corp.

P.0. Box 5837, MP 184

Orlando, FL 32855

(305) 356-2086

Mr. Hal Greenhouse

Allied Bendix Aerospace

1300 E. Joppa Road

Baltimore, MD 21204

(301) 583-4102

Ms. Diane Feliciano-Welpe

Oneida Research Services, Inc.

One Halsey Road

Whitesboro, NY 13492

(315) 736-3050

Mr. Thomas F. Gukelberger

IBM

B/908

Poughneepsie, NY 12601

(914) 432-7238

Mr. Victor Fong

Paname tries

221 Crescent Street

Waltham, MA 02254

(617) 899-2719

Mr. I. Macit Gurol

Vi shay Intertechnology, Inc.

63 Lincoln Highway

Malvern, PA 19355

(215) 644-1300, x371

Ms. Kathleen V. Frisbie

DOD, Attn: YZ53

9800 Savage Road

Ft. Meade, MD 20755

(301) 688-7739

Mr. James W. Guthrie

Sandia National Laboratories

Division 7551, P.O. Box 5800

Albuquerque, NM 87185

(505) 846-4266

330
Mr. Ray F. Haack

Jet Propulsion Laboratory

California Institute of Technology

M/S 125-214

4800 Oak Grove Drive

Pasadena, CA 91109

(818) 354-6568

Mr. Orjan Hallberg

RIFA AB

SI 638

Stockholm, Sweden SI 681

(08) 757-4626

Mr. George Harman

National Bureau of Standards

Building 225, Room A331

Gaithersburg, MD 20899

(301) 975-2097

Mr. William L. Harrington

RCA Laboratories

David Sarnoff Research Center

Princeton, NJ 08540

(609) 734-2415

Mr. Sab Hasegawa

National Bureau of Standards

Building 221, Room B31

Gaithersburg, MD 20899

(301) 975-2620

Mr. Howard J. Herzog

Spectra-Physics, Laser Analytic Div.

25 Wiggins Avenue

Bedford, MA 01730

(617) 275-2650

Ms. Laura A. Hughes

Rockwell International

3370 Miraloma Ave (DF03)

Anaheim, CA 92803-4192

(714) 762-2357

Ms. Karin Hulten

MIMX, IC-Division, RIFA AB

Isafjordsgatan 10-16, Kista

S- 16381 Stockholm, Sweden

(08) 747-4920

Mr. Ronald Hunadi

Furane/Rohm & Haas

5121 San Fernando Road W.

Los Angeles, CA 90039

(818) 247-6210

Mr. Douglas K. Ishio

Li tton/Amecom

5115 Calvert Road

College Park, MD 20740

(301) 864-5600, xl 066

Ms. Charlotte Jackson

Northrop Electronics

P.0. Box 5032

Hawthorne, CA 90251-5032

(213) 600-7196

Mr. Kevin Jewett

ILC Data Device Corp.

105 Wilbur Place

Bohemia, NY 11716

(516) 567-5600

Mr. Robert J. Kalin

Medtronic, Inc.

7000 Central Ave., N.E.

Fridley, MN 55432

(612) 574-4237

Mr. Didier Kane

RADC/RBRE

Griffiss AFB, NY 13441-5700

(315) 330-4055

Mr. James W. Kersey

Sandia National Laboratories Livermore

P.0. Box 969

Livermore, CA 94550

(415) 422-2740

Dr. J. A. Koprio

Balzers AG

ABT ELA

FL9496 Blazers

Principality of Liechtenstein

Mr. Michael Leek

VG Gas Analysis

Aston Way, Middlewick

Cheshire, England CWO OMT

331
Mr. Jim C. Lawson

Teledyne Microelectronics

1 2964 Panama Street

Los Angeles, CA 90266

(213) 822-8229

Mr. Louis Liang

Fairchild Semiconductor Corporation

313 Fairchild Drive; M/S 4-390

Mtn. View, CA 94039

(41 5) 962-4164 or 4166

Ms. Patricia A. Long

Hughes Aircraft Company

P.O. Box 11337

Tucson, AZ 85734

(602) 295-5230

Mr. Robert K. Lowry

Harris Semiconductor

Box 883, MS 62-07

Melbourne, FL 32901

(305) 724-7566

Mr. Aaron Der Marderosian

Raytheon Co.

528 Boston Post Rd.

Sudbury, MA 01776

(617) 440-2791

Ms. Margie Marlow

Watkins-Johnson Co.

700 Quince Orchard Road

Gaithersburg, MD 20878

(301) 948-7550, x535

Mr. Craig B. Mauldin

General Electric Co. - NDD

7887 Bryan Dairy Road

Largo, FL 34294-2908

(813) 541-8811

Mr. Ralph McCullough

Texas Instruments

P.O. Box 65501 2, M/S 1 7 Dallas, TX 75265

(214) 995-3931

Mr. Benjamin A. Moore

RADC/RBRE

Griffiss AFB, NY 13440-5770

(315) 330-4055

Mr. Jay Mucha

AT&T Bell Labs

600 Mountain Avenue

Murray Hill, NJ 07974

(201) 582-3659

Mr. Robert Nearhoof

RCA/Solid State Division

Route 202

Somerville, NJ 08876

(201) 685-6465

Dr. C. A. Neugebauer

Corporate Research and Development

General Electric Company

P.O. Box 8, KWC-1603

Schenectady, NY 12301

(518) 387-5399

Mr. John A. Olenick

Westinghouse Electric Corporation

P.O. Box 746, MS 496

Baltimore, MD 21203

(301) 765-2259

Mr. Ronald Pacheco

Digital Equipment Corporation

100 Minuteman Road AP02/F2

Andover, MA 01810

(617) 689-1482

Mr. Carl T. Pallister

Martin Marietta Denver Aerospace

P.O. Box 179

Denver, CO 80201

(303) 977-4127

Mr. W. C. Pei la

Sandia National Laboratories

Division 8153

P.O. Box 969

Livermore, CA 94550

(415) 422-2077

Ms. Maxine B. Pennington

Allied Bendix Aerospace

Bendix Kansas City Division

2000 E. 95th St., P.O. Box 419159

Kansas City, MO 64141-6159

(816) 997-4187

332
Mr. Wolf Penzel

Honeywell, Inc.

MN1 1-1812

600 2nd Street NE

Hopkins, MN 55343

(612) 931-6983

Mr. John C. Pernicka

Pernicka Corporation

1 1 2 Racquette Drive

Fort Collins, CO 80524

(303) 224-0220

Mr. Daniel J. Peters

Allied/Bendix Corporation

Kansas City Division

P.O. Box 419159

Kansas City, MO 64141-6159

(816) 997-5507

Mr. David A. Pinsky

Raytheon Co.

Hartwell Road

Bedford, MA 01730

(617) 274-4252

Mr. Greg Pitts

M.C.C.

12100 Technology Blvd.

Austin, TX 78727

(512) 331-6200

Ms. Bernedette Preston

Department of Defense

Y253

Ft. Meade, MD 20755

(301) 688-7739

Mr. Brian E. , Reilly

A1 lied-Bendix Aerospace

P.O. Box 419159

Kansas City, MO 64141-6159

(816) 997-3564

Mr. Stephen P. Rogers

Motorola

5005 E. McDowell Road, M/S PI 66

Phoenix, AZ 85005

(602) 244-3711

Mr. Jack Ronning

Medtronic, Inc.

7000 Central Avenue, NE

Fridley, MN 55432

(612) 574-4761

Mr. Thomas J. Rossiter

Oneida Research Services, Inc.

One Halsey Road

Whitesboro, NY 13492

(315) 736-3050

Mr. David Ruben

Motorola

Maildrop HI 02 3 8201 E. McDowell Road

Scottsdale, AZ 85252

(602) 481-8166

Mr. Philipp Schuessler

IBM/FSD

M/S 0570

Owego, NY 13827

(607) 751-4219

Mr. Stephen D. Senturia

Massachusetts Institute of Technology

77 Massachusetts Avenue, Room 39-567

Cambridge, MA 02139

(617) 253-6869

Mr. Glenn Shirley

Intel Corporation

145 S. 79th Street

Chandler, AZ 85226

(602) 961-5357

Mr. Ed Smith

Olin-MRL

91 Shelton Avenue

New Haven, CT 0651 1 (203) 789-5293

Mr. Robert J. Straub

Delco Electronics Corporation

1800 E. Lincoln

Kokomo, IN 46902

(317) 451-7027

Mr. Charles T. Taverner

Bell Aerospace Textron

P.O. Box 1, MS C-65

Buffalo, NY 14240

(716) 297-1000, x747

333
Mr. Philip R. Troyk

IIT

10W. 32nd Street

Chicago, IL 60616

(312) 567-5324

Mr. Mark Wilson

Blazers

8 Sagamore Pk • Rd

Hudson, NH 03051

(603) 889-6888

Mr. O. Manuel Uy

Applied Physics Laboratory

John Hopkins University

Johns Hopkins Road

Laurel, MD 20708

Mr. John Yim

National Semiconductor

2900 Semiconductor Dr.

Santa Clara, CA 95057

(408) 721-3870

(301) 953-5334

Mr. Tom Walker

Mail Code 6051

Naval Weapons Support Center

Crane, IN 47522

(812) 854-3828

Mr. David Wall

Spectra-Physics, Inc.

,

Laser Analytics Div.

25 Wiggins Avenue

Bedford, MA 01730

(617) 275-2650

Ms. Debra Waller

Digital Equipment Corp.

1 00 Minuteman Road

Andover, MA 01870

(617) 689-1552

Mr. Joel Weiner

Johns Hopkins Applied

Physics Laboratory

1 6-209

John Hopkins Road

Laurel, MD 20707

(301) 953-5000

Mr. Steve Weisskoff

Phys-Chemical Research Corporation

36 West 20th Street

New York, NY 10011

(212) 924-2070

Mr. David Wilson

Martin Marietta

P.0. Box 179

Denver, CO 80201

(303) 977-3838

334

* U.S. .GOVERNMENT PRINTING OFFICE 1987; 181-100/62515
HBS-114A (REV. 2-80

U.S. DEPT. OF COMM.

4.

1 . BIBLIOGRAPHIC DATA

SH EET (See instructions)

PUBLICATION OR

REPORT NO.

2. Performing Organ. Report No. 3.

NBSIR 87-3588

Publication Date

July 1987

TITLE AND SUBTITLE

RADC/NBS International Workshop on Moisture Measurement and Control for

Microelectronics (IV)

5. AUTHOR(S)

D, Kane, B.A. Moore, and E.J. Walters, Editors

6. PERFORMING ORGANIZATION (If joint or other than NBS

see instructions) 7. Contract/G.rant No.

NATIONAL BUREAU OF STANDARDS

U.S. DEPARTMENT OF COMMERCE

GAITHERSBURG, MD 20899

8 . Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City. State, ZIP)

10. SUPPLEMENTARY NOTES

~| Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bi bl iography or literature survey, mention it here) This fourth Workshop on Moisture Measurement and Control for Microelectronics

served as a forum on moisture and/or materials reliability problems and on ways to

control them or measure their extent. Twenty-two presentations are included which

contain detailed information on hermeticity measurement and definition; development

of standard packages for mass spectrometric calibrations; moisture interaction

with various materials; and techniques that can be used to measure moisture microelectronics. It was clear from several presentations in this workshop that a very systematic approach is needed when organic materials are involved; all the

variables must be identified and studied one at a time. This is the key to lotto-lot reproducibility, materials selection, and control; hence a better reliability

at the design phase will decrease the need for testing, hence the cost, thus

resulting in a greater satisfaction to the customer.

12. KEY WORDS (S/x to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) Analysis of moisture content; hermetically packaged semiconductor devices; hybrid packages; mass spectrometer measurements; moisture; organic package materials; moisture

generators; moxsture sensors; quality control; reliability of semiconductor devices;

semiconductor devices.

13. AVAILABILITY 14. NO. OF

PRINTED PAGES

|XH Unl imited

| | For Official Distribution. Do Not Release to NTIS 342

I Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402. 15. Price

[jQ^Order From National Technical Information Service (NTIS), Springfield, VA. 22161 $30.95

U S COMM- D C 6043-P80
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